that's not even a competition--the 9600XT is what, seven months newer than the 5600U? The 5600U is being phased out in favor of the 5700U, but the 5700U is still around $180. for $140... damn, you are not going to beat that 9600XT, especially since it has a HL2 voucher. even the 9600 Pro was neck-and-neck with the 5600U, so the 9600XT should beat it quite handily.
while I prefer NV cards because I've found their drivers to be more stable (in general usage, but I've only had one crash-to-desktop ever with this 5700U, so probably in games too), you are not going to get a better card than the 9600XT for that money. jeez, that's crazy, that thing was going for ~$200 a month ago.
if you want to spend a bit more money, you could find a 5900SE/XT/non-Ultra/whatever for $190, and that should beat the 9600XT in just about everything, including a lot of PS2.0 applications, but it's $50.. that's an amazing price on the 9600XT. heh, Newegg has it for $175--that's a deal.
still, with either one, though, you're going to be quite limited by your processor, motherboard, and RAM. I guess that's the next thing to upgrade..