It would be really great if you could mod gaming cards for workstation use. Shopping for workstation cards is a depressing process. I can get a huge honkin' gaming card that takes up two slot spaces for about $250, but the same hardware with workstation-specific drivers costs upwards of $1200. I've read that people used to flash the gaming cards' BIOS to mod them into workstation cards, I wonder if that's still possible....
The cost of getting a low-end chip for a bunch of money is that the workstation cards have their own drivers, tweaked completely differently than your gaming drivers. You're not just buying a card, you're buying a software support infrastructure for that card. IMO, there's no need, if you can't be assured that the GPU or VRAM will be a serious limit, to go overboard with it. IE, built the system with a very nice CPU (90% of the time, planning around a CPU upgrade is useless); then for the rest of the performance parts, be conservative, thinking about how you'd upgrade it.