T-Break posted a review on the ATI X1950 XTX Video Card

The core on the X1950 XTX almost remains unchanged running at the same frequency as the x1900- 650MHz. ATI does say that the core is tweaked and thus the model number has changed from R580 to R580+ and from our talks with ATI, that + sign is basically an updated/tweaked memory controller that will also take over the x1900 series in the future.

ATI X1950 XTX Video Card Review