The only thing to look at is the memory.
If the R9600XT memory is good (like 2.86ns Smsung [which is better than the 2.8ns Hynix]) then it will be a far better card.
The core on the R9600XT will almost always have about a 50mhz overclockability boost above the pro due to the benificial prperties of the LOW-K/D insulation in the mfr process.
However sometimes the R9600XTs are saddled with crap memory (especially from people like Sapphire), and then the OC benifit of the core is negated by the terrible OC'ing memory.
My R9600Pro will run 535/365 all day/night. Most R9600XTs should be able to reach 550-575 on the core without tourble. However if they are saddled with the 3.3ns crap memory then they likely won't go mcuh higher than 315mhz. And since the R9600 series is so highly memory constrained you really want to make sure you know what you are getting if you are overclocking. If you weren't overclocking then the R9600XT would be the obvios choice.
Now the price of a BBA R9600Pro makes it an attractive option. In my experience the Built By Ati R9600Pros overclock among the very best. As I said my R9600Pro does 535/365 no problem, and will do brief (like 1hr) runs at 540/370.
Check the R9600XT and if it has the better memory, I'd definitely go with that. Especially with some of the added features some of them have. I'm not sure if the Sapphires are the ones with or without thermal sensors, which would be handy for overclocking.
With your system I'd try and get the very most out of it I could so the R9600XT would be nice. Of course if you can save a little money and put it towards a new card later, and get like and X800SE or GF6800XT/SE (depending on what tyhey look like) at an earlier replacement date, then that too might be a good idea.
As always, that's just my two frames worth.
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