I never spend more than five hundred dollars on my machines.  But they are only dual core AMD boxes.  I get my parts from Fry's Electronics, newegg.com and tigerdirect.com.  Carrara 5, 6, 6 Pro, and 7 Pro run fine on both my boxes.  I have 3 gigs of memory on one, and 4 on the other and run XP-Pro 64 bit.  I have NVIDIA GeForce 7300 LE video cards.   NVIDIA cards let you choose the options to run each of your programs.   I have written a tut about doing that here.

Free men do not ask permission to bear arms!!