The direct compare for the 7900 GT is the AIW X1900, which basically is the "XL" version of the X1900 series, without the XL letters themselves. The X1800 XL doesn't have the extra pixel shaders, and the AIW X1900 got them. Not to mention that a decently over-clocked (+40 Mhz Core & +40 Mhz Memory, minimum, which can be achieved extremely easily (we all know ATi's X1800/X1900 series are over-clocking beasts in general) AIW X1900 will actually be better than a stock-speed 7900 GT anyday.
I know however that a decently OC'ed 7900 GT offers superb performance, and with its price getting more affordable almost on a per-month basis, makes it an excellent choice still.