I been playing around wif the ATI Tool and 3DMark also... I actually very dis-sastify with my X1300 Pro, cos during purchase time I decided to spend extra on Processor and hold back on the GC (Since I use it for media centre).

My GC core is 600, memory is 400. So far for stability i tried:
620/420 No artifacts
630/430 Dam alot of artifacts

But as a 4-pipe 128 bit card I don't think you can see significant improvements.