All I can say, is look at the deals on fatwallet.com. i7s aren't as far out of your price range as you'd think, given how many components of a PC you already have. Are workstation cards better for the type of stuff you do, or gaming cards? I work with cad, and the performance increase from a gaming card to a cad card is absolutely phenomenal, even when the specs of a $150 cad card look like a 2-year old $50 gaming card.
Also beware of multicore/multithread benchmarks vs. single-threaded stuff.
