Well, I don't honestly see why people under-rate the X1600 series video cards. I mean, they have really good specs for their price, not to mention, you don't need a Master card to have them in CrossFire. Plus, the 7800GS is in AGP interface, which is slow compared to PCI-Express, which is twice as fast as AGP.
So the X1600XT has more memory, depending which model you want, it also has the ability to use a dual-GPU interface. CrossFire is really nice to have with the X1600 cards. If I was to buy a 7800GS or an X1600XT. I'd take the X1600XT, actually, make that TWO X1600XT's, because I'd buy two X1600XT's for the price of one 7800GS. You just can't beat the price of the X1600.