I'd get the 7900GT KO Superclocked, which, at $270 (after rebate at newegg), is $80 less than the 7900GT Signature, and $180 less than the 7900GTX. The core and memory speeds are both 20 MHz less than those of the Signature.
Don't by an EVGA video card. Iknow you have two choices, but if you can under whatever constraint go for an X1800XT. If not well, the Signature Series will save you money and still over some good performance.
The signature series has some advantages, some special support line when you call, higher clocks of course. The only difference besides money would be the additional memory on the GTX and huge heatsink. Either one of those have enough power and cooling to keep you happy. Oh and I'd advise against Always Technically Inept or ATI as some call them. Seems someone outside of ATI has developed some Omega brand drivers for the ATI cards and they out preform what ATI's own can code, think that tells the story pretty well.