Don't buy a/an HP. I recommend custom building one. Don't get Vista. Stick with Windows7 or WindowsXP if you're going for a Microsoft feel. I recommend getting a Quad Core Processor unless purchasing the i7's, which is physically 4, virtually 4, totaling an 8-Core Processor. If running XP, stick with 4GB of RAM as they can only handle about 3.25GB's. If you are going to settle for Windows7, you should get about 12GB's of RAM running it on a 64bit O/S. For the GFX Card, I recommend getting 2x 1.5GB's of whatever brand is your preference. Hard drive space and number is user's preference..
I recommend:
Windows7
Power Pack - 900W
Processor - i7 960, 3.2GHz, 8MB cache, 4.8GT.
Memory - 12GB DDR3 (6x 2GB) on 64x O/S.
Graphics Card/s - 2x 1.5GB in your choice of brand.
Hard Drive Setup - 2x 1.5TB Internal.
Motherboard - Anything, as long as its DDR3 capable. It would benefit you if it contained an integrated 512MB-1024MB GFX Card for added performance.
This is in no way going to be cheap. The GFX Cards alone will set you back about $1,500 depending upon the brand. If you choose this road, make sure to have ~5 fans inside this thing. I recommend having cooling gel implanted into the system. Its liquid cooling and, to date, the most effective cooling method. Liquid cooling triumphs over air cooling in vehicles and therefore has been implemented into computer systems (obviously using a completely different method but still uses the liquid formula).
Good luck!