I just finished purchasing for a computer, but all my equipment is still on its way. If you don't intend on building an intel system, don't listen to me, I've only ever worked with intel.
BTW, my sole experience is not in buying this last computer. The computer I have now I built myself. I do volunteer tech support work at my school as well. I do, to some extent, know what I am talking about.
Always buy a full tower. Smaller towers just don't do the job. Figure $80 -- $200+ depending on what you want, especially if what you want is fans / aluminum / PSupply etc.
Buy the biggest PSupply you can, at least 450w. Figure $100 - $150 for one, I think.
For Intel Mobo's, buy from Abit, Asus, or IWILL. IWILL actually specializes in worstation boards and is where you'd want to go for DP systems. Figure $100 - $250+ depending on the chipset 850 or 7505, the company, the RAM type, etc. There is a lot to consider on a Mobo, you need to know what kind of RAM, probably RAMBUS or DDR, you intend to use, what processor you want to use, in this case P4 or P4 XEON for DP machines, and what you want on board, auido, lan, RAID, SCSI, etc. Make sure it has AGP 8x though, or your setting yourself up to buy a new one within the year.
P4 vs P4 Xeon -- When I was buying a new system, I really wanted to know what the difference between these two was. The only thing I ever turned up is that Xeon processors are designed for Workstations and Servers, but all the specs appear to be the same for P4 and Xeon. Note that to my knowleadge, there are only DP Xeon Systems. The price difference in P4 and Xeon is nonexistent. Make sure you buy proccessors with 533mhz front side bus, instead of the old 400mhz models. You'll wonder why they're so cheap. . . Figure $250+ for a good processor. 3.06ghz can cost near $700!
Hyper-Threading-A new, to consumers anyway, technology that makes 1 processor act like 2 logical processors. If you want a DP machine, buy processors with Hyperthreading and save on the second processor. Note that only W2000 and WXP support DP systems and that (to my knowleadge) only XP supports Hyperthreading. Processor costs vary from around $200 to $600+ dollars. To make it easier, the fastest processor around at the moment is 3.06ghz, at more than $600.
RAM--RAMBUS vs DDR -- More Mobo's use DDR, simple as that. RAMBUS (RD) IS faster, but very few motherboards actually use it. I currently have RD ram, but am getting DDR because I couldn't find a good Mobo that supported RD. I think you need at least 512mb, I'm going for 1gb, and I would suggest 2gb if you have the money. RAM can be the sole determining factor of how well your big games (Morrowind and OFP, to name 2 that are out, BCG to name one we should all know) run. RAM will cost you about $.75 a meg at the moment. Note that AMD systems will only ever support DDR.
RAID-If you want extra space without compromising speed and while still using IDE HD's, RAID is probably the way to go. For a gamer, RAID essentially allows you to take two smaller harddrives, say 40gb and run them as if they are a double-fast 80gb HD, the bigger your harddrive, the slower your read time, and RAID helps to cut down on this.
SCSI-Fast alternative to IDE, never used it, but know that it is significantly more expensive to buy SCSI equipment.
On Board. . .-- The only technology that I think should come on board is your LAN. Buy a Mobo with Gigabit Lan, and you won't need to worry about a LAN card.
Hard Drives -- If you're looking for SCSI, I don't know anything. Otherwise, buy a Maxtor or Western Digital Harddrive. They make good stable products. Figure no more than $100 for a 40 to 60gb hard drive. Make sure it's 7200rpm.
Video Cards -- I'd say this is primarily opinion based. ATi's new card (9700) has plenty of advantages, but I don't trust ATi's drivers. I still use nVidia, and if you wait long enough, you can get a card with the nV30 chipset that should be faster than the 9700. Figure $250+ for a good video card, either the 9700 or a geForce4 Ti-4600.
Sound Cards -- The newest and best consumer card is the Augdigy 2. If you've already got an Audigy card, keep it, if you have anything else, buy an Audigy 2. From a retailer, it will cost $200.
Optical Drives -- You probably want a good 16x DVD drive for your computer. This will be less than $100. After that, the issue of a CD-RW or even DVD-RW arises. If nothing else, buy from companies you know: Philips, Sony, etc. They will provide you with the stuff most likely to work. Note that there are multiple DVD-R and DVD-RW formats out at the moment. Buying from a major company will increase your chances of buying a drive with the format that will survive. Otherwise, buy a CD-RW and hold off on the DVD-RW. A CD-RW should cost between $100 and $200 depending primarily on the company.
Monitor -- Plan on a 19" Flat or Flatscreen monitor. I would suggest Flatscreen over Flat only because the refresh rates on Flat LCD monitors is lower than on CRT's and *can* cause streaking in very fast games. Look for a dot pitch of .20mm, these are the better monitors. Figure between $150 and $300+ for a monitor.
Keyboards / Mice -- I've always found that keyboards in the 2nd level of cheapness (around $30 from a retailer, not $15!!) work better for gaming than the higher level ones, unless you're into wireless and all that crap. . . I do not believe in wireless keyboards. Microsoft's keyboards always works great with Microsoft's OS. A digital, USB mouse, perhaps even wireless, is a must for gaming. Try to get one of the new ones with dual sensors for enhanced accuracy, they reduce the chance that your mouse will skip, a slight problem when the difficult to clean lenses get dirty. Figure $20 to $50+ depending on what you get. $20 should be a fair Logitech mouse that you can buy anywhere.
I think that covers everything. If you want to shop for AMD stuff, the same rules apply for everything but the Mobo and processors. Oddly enough, a lot of computer buying is opinion since even the $700 machine some fool buys from Best Buy with a $699 rebate will run most games, it's really how well you want your games to run, for how long, and at what resolutions. For games like BCG, Morrowind, OFP, Unreal2, and Microsoft Flight Sim, high level computers are a must. This is all I know. I hope that everyone will disagree with me and tell you what to buy instead, that way you'll have more choices. I hope that I have not told you what to buy, but suggested how you should buy it.