Stock is 500/1000 right and you're doin 561/1090?
Hmm...what was your original score in 3Dmark03 do you have it recorded by chance?
My memory on my current card go only go up like 50 but my core can go up like 100..
Seems like 600/1100 should be possibly with that card, but then again I don't know what kind of cooling XFX has on that, meaning the quality of the heatsink on there.
Hopefully I'll be getting an eVGA 6600GT here in a couple weeks and I'll have something to compare to yours
Other than that if it seems everything is smooth and cool (which btw did you check your idle and load temps before and after overclocking?) if you still have low temps then keep pushing unless you're not comfortable doing so