if you're dealing with high amounts of polygons, it's going to be more CPU dependant, although it will probabbly pay to have a good graphics card aswell.
my advice is to get a socket 939 AMD system, with an X2 3800+ to 4800+
your system will last for a long time before you need to upgrade if you do get a 4800+. it is currently the best CPU commercially available, except for the Opteron 850's and above, although they cost heaps, and are designed more for servers.
for a motherboard, either get a DFI NF4 Ultra-D, or an MSI K8N Neo4, depending on how much you want to spend. the DFI is a better board, the MSI is not quite as good, but less expensive.
video card: very recently, Nvidia has released a 6800 GS, which doesn't cost much yet has great performance.
RAM: OCZ 2x512MB or 2x1GB. 2GB total will give you a more future proof computer. 1GB will suffice, but ATM more people are starting to move over to 2GB for high end systems. it is probabbly the better option with the upcoming games.
for hard drives, look for Seagate. thet make some of the best drives around, which are fast, quiet and really reliable.