you probably wont notice too much (if any) of a difference above about 2.7
you wont be bottlenecking an 8800gt running it around that speed, or a little above. not enough to notice anyway....not like you had it running with a pIII or something lol.
the X2's performance levels out if they are overclocked to around 3ghz, typically.
they will show higher benchmark numbers occasionally, but it seems to me that mine have always perfromed best OVERALL clocked between 2.9 and 3.3
meaning you can clock it higher (maybe), but usually temps from the high voltage needed cancel out any improvement of running it faster than that. unless you have the chip that wildside and i have...
with it, voltage usually isnt an issue.
EDIT: just saw the abpve post. whats your ram set at? also, i wouldn't expect to get too far with stock voltage.
my old 4200 windsor was cookin and it would barely run over 2.9