Wow, thats a nice budget(under 6k?). I would get at least quad core, 4 gig of ram, an 8800 Nvidia card, 2 HDD(300gb or higher) two or three 20" LCDs or higher(3 are awesome for 3dcg and games), 64bit OS to support larger amount of ram(Vista is gaining 64bit support alot now) etc... this would be my base computer to start out with with the right budget in 3dcg, which can always be upgraded later depending on MB.

For me, the monitors and CPU would be the first things to look at. Fast video card if you want high end games and mega polygon models for OpenGL viewports etc... 

I run my very outdated computer as a dedicated 3dcg one, and found it is MUCH less hassle to work on(its not connected to internet for less services in background etc.). 

my .02