It's a tough call to make. The 5900 should be faster in DX8-and-lower games, should do well in Doom 3, and may have better-looking AF. The 9600XT may in fact be faster in DX9-level games that make heavy use of shaders, may in fact do better than the 5900 in Half-Life 2 (and includes the game, thus making the price of the card somewhat less than what you pay), and has better-looking AA. Fundamentally, the 5900 is a high-end card, and the 9600XT, mid-range. But the real question is what games you (will) play, and which card will serve you better in them. I think it's a tough call between a $230 5900 and a
$180 9600XT. Unfortunately, I can't personally tell you how the 5900's AA stacks up compared to the 9600, as that would be a large deciding factor for me. There's no point in buying a 5900 if you won't use AA+AF or if the AA isn't nice, IMO. But I suppose the resolution you'll be playing at is also a consideration. Don't expect decent framerates w/4xAA 8xAF out of the 9600XT much beyond 10x7; the 5900 should be able to provide equally-smooth framerates at 12x10, and possibly higher, because of its much greater memory bandwidth.
There are plenty of reviews for you to compare benchmarks between the two cards in games and resolutions that matter most to you. I suggest doing that first. You can start with AnandTech's recent roundups to find 5900 vs. 9600XT numbers. Firingsquad should also have some numbers, as will GamePC.
Edit: In terms of speed, the 5900 wins, and the 9600XT won't be able to compete as a high-end card. My main concern, HL2, is really a fine point: I'd like fast DX9 performance, but the 5900 should at least offer good DX8 performance. As the 5900 should be faster all-around, it may be a better buy right now. But if you factor in the cost of HL2 (say $40, to account for the $10 charge to mail you the game CDs), the 9600XT will cost almost half of what the 5900 will, so the two cards aren't really in the same league. (As I will definitely buy HL2, I factor it in the purchase price.) The 5900 is really meant to compete with the 9800P, though the Pro is currently priced higher (even factoring in HL2), probably due to supply and demand.
Really, your main consideration should be which card will provide you with acceptable speed at the settings you want, with IQ being a major, but lesser, factor. I think a 9600XT is a great deal at $164, but if it's not fast enough in the way you'll use it, then there's no point in buying something you'll be unsatisfied with from the beginning.