Carl,

I have a Athlon 3200+ with 1 gig of memory with an ATI 9800 pro video card. I am running the sim at 1600 x 1200 and am getting around 40 fps on average with most everything turned on.

Your system should be OK. The only area that may be worthwhile upgrading would be your video card. I would suggest running the sim under your current configuration and seing how well it performs and if you are happy with that performance. If you want more eye candy, upgrade your video card.