^Actually OP can spend money on a workstation GPU or softmod a normal gaming card in to one which would help quite a bit in Maya, Rhino, 3DS and other 3D modeling or CAD programs. Not to mention if OP's using CS4 he can get GPU acceleration for filters,etc. (Note: Must be running Vista x64/x86 or XP x86; Won't work with XP x64)
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My recommendations:
Q6600 (or other quad core)
P45-UD3P or P45-UD3L or P45-UD3R (Depending on if you need RAID, CrossFire,etc)
PC Power & Cooling 610 or Corsair 400CX or other good quality PSU from Antec, OCZ. DO NOT SKIMP ON PSU
WD 640GB (or you can re use the 160GB)
2*2GB DDR2 800 GSkill PI (or even 8GB)
Vista or XP x64 (You MUST run a x64 OS to use more than 3.5GB)
(Note: Be careful with XP x64, you may run in to not having XP x64 drivers for a particular device ie printer)
Video card is a tough choice. You can do two things:
1. Get a regular gaming card that can be softmoded to enable the workstation drivers to run on it (ie 8800GTS to Quadro 4600) (I would personally do this)
2. Buy a workstation card, which is quite expensive.
Also any chance of upping the budget to $500-550?
Total cost for Q6600 + P45-UD3L + 640GB + 4GB DDR2 800 G Skill PI is $423 already. So with a Corsair 400CX (after MIR) is $464.