JJJ, I always had this feeling you were a [closet] gamer! Must of been your love for Penny Arcade that gave it away. Anyways...
Couple of review sites:
I suggest investing in a 20"-24" wide screen monitor capable of handling 1680x1050. I bought this one a few weeks ago and boy, Wide screen gaming rules. An absolutely essential upgrade-especially from 1280x1024. Although, one caveat exists: in order to support this resolution, one needs to have powerful graphics card.
A good pair of Cans/Speakers + sound card helps as well. If you listen to music/game frequently, dedicated hardware is simply superior to on board proprietary hardware.
5000 dollars is a lot of money. And a lot of the suggestions Athenian proposed are a bit overkill. You can make a bitchin' computer with 2000 easy. Hell, I built mine a few months ago for 1300.
DD3 memory, whilst the future, is ridiculously overpriced for the gains you receive and hardly any motherboards support it. DDR2 is dirt cheap right now and you could buy nearly triple the ram you could with DDR3. 2gb is pretty standard these days if you want a satisfactory amount of ram (only windows applicable) If you plan to run xp, the kernel caps out at 3.5gb because it's a 32-bit operating system. With the 64-bit version of xp / Vista, I think it's 16gb. Although, one caveat for 64-bit OS': driver support is worse than 32-bit - though it is improving significantly. In other words, 32-bit = 2gb & 64-bit = 2gb+.
ATI/Nvidia, you can't go wrong with either of them. Nvidia has better linux support though.
Raptors are fast. Really fast. But they're loud. Like RAM, hard drives have become extremely cheap. I'd pick up a Sata2 one with as much space as you want.
Intel/AMD, well Intel is clearly winning the benchmarks. AMD is still a good choice, especially if you're looking for something not as expensive. If you go Intel, definitely try to procure one of the new 45nm chips.
I feel most people misgauge how much power their system needs. This was the last thing I picked out. Research the required Amp rails needed to supply your graphics card.
I can not stress the importance of having a quality power supply and RAM.
If you have any other questions, feel free to shoot me a PM or reply here.