You're right, it is rated at 600mhz. Have you tried putting arctic silver 5 and some ramsinks on? Could be your memory is not getting the cooling it needs... even with the stock "aluminium sheet" cooler on the back i got 590mhz mem with as5...
Update on the new GTO2... as mentioned earlier, it's got dual-dvi and and VIVO, R480 core, 1.6ns GDDR3, and 16 pipelines out of the box! :) Will post a pic soon :)