The big difference with Graphics cards for combustion is what Open GL functions it supports. For example I went from a Geforce FX 5600 128MB $120 (which seemed ok on everything but particles) to the “Low End” of the “Workstation” Cards by Nvidia the Quadro FX 10000 128MB $420 (ebay) and my particles went from crawling to almost realtime. I was blown away. The only real difference in those cards was the Open GL capabilities. Remember that combustion doesn’t need High Poly capabailties. How many polys do you really have in even the most complex 3D composite… Not many. Texture memory and open GL. Discreet has a sort of outdated list of support cards but I suggest getting an older Quadro or Fire GL card for the open gl support rather then buying the latest gaming card expecting great performance boosts.
Beyond that it seems framebuffer hardware and the like is the other way to increase performance and TONS of RAM obiously.
Thats my 2 cents.