About 2 months ago, I performed a major core system upgrade (CPU, MB, RAM, VC). I was planning on getting an R9600XT, and was interested in the low price of the 256MB version. However, everyone I talked to, told me that 256MB on an R9600 is useless, since the card itself can only address 128MB worth. The R9800 series on the other hand (except for an Atlantis model, can't remember which manufacturer), can address the whole 256MB worth. Now, the R9800 Pro you mentioned was only 128MB, so if the R9600XT can only use 128MB, then the R9800 Pro should actually run faster, since its got the better core and wider data pathways.
Answer, if you can afford it, go with the 9800Pro, even if its 128MB. If you can't, stick with the R9600XT with 128MB, which is what I'm running.
Hope that helps.
Later!