With no real budget, the skies the limit. I don't have much time right now to exhaustively list what should be included, because I'm rushing to get things done before leaving town tomorrow, but, if I had a nice budget and was to build a kick ass PC right now, this is what I'd get:
Intel Core 2 Quad Q9650 ~$560
ASUS Rampage Extreme (X48-based) ~$400
2x Radeon HD 4870 X2 ~$1140
OCZ Reaper HPC 4GB (2 x 2GB) DDR3 ~$250
Western Digital VelociRaptor 300GB ~$300
Seagate Barracuda 7200.11 1TB ~$170
2x DVD-Burner ~$55
SilverStone TJ10 Chassis ~$330
PC Power & Cooling 1200W ~$530
That right there would run you about $3,735 before taxes, and exclude a keyboard, mouse, OS (Vista Ultimate 64-bit is a good choice) and monitor. Those purchasing decisions would be more of a personal taste. But if you really have money to spend, then you need a 30" display to really push those GPUs. I own a Gateway XHD3000 and love it, but some prefer the richer color of Dell's 3008WFP, so you really should read up on those, since they are not cheap purchases.
That computer could be future-proof also, by adding in a second kit of the same RAM, to make 8GB. You could save some money by scaling down to a SilverStone DA1200 PSU, which runs around $300. PCP&C is just generally known as having the best PSUs in the industry, but I have the SilverStone here and it works amazingly.
The CPU is a good buy, because just last week, the same-specced QX9650 would have cost you just about $1,000. The higher-end QX9770 is an option too, but you'd really be throwing your cash away. You'd essentially be paying $900 more for an extra 0.2GHz. You can get the Q9650, put some great water-cooling on it (I can't recommend that, someone else can I'm sure), and overclock it to 3.8GHz or 4.0GHz without much issue.
You could also add in a Blu-ray drive if you wanted, or a second 1TB hard drive for even more space. The VelociRaptor should be used only for the OS and your games... the Seagate drives would be used for everything else. The VelociRaptor is much faster than those (hence the high price and lack of density), so it's perfect for your games.
I am not sure if I'm skipping over anything, but hopefully someone can fill in any gaps I left.
