The 7950 is the faster card. The 7800GTX 512mb is slower than the 7900GTX and the 7950GX2 is actually like two 7900GTX cards put together or you can say the speed would be similiar to a 7900GTX in SLI. Also the 7800GTX 512mb never really went into full scale production. Only a few thousand(I think it was less than 10,000 of these cards were made.) The only reason why this cards came out was to compete with the x1800XTX until the 7900GTX came out.