well, Sudheer Anne was a little harsh...
anyway...the 5.5ns cards are spec'ed to run at 366mhz (or 363mhz?) so 390mhz is a decent amount higher. that difference would be like a geforce2 gts running at ~355-360mhz (the amount of overclock, not the speed). it is not a huge overclock but it is still pretty good. the upper limit on the 5.5ns cards is probably around 400mhz and results always vary a bit with overclocking. also, even houh you can go higher without locking up, once you see artifacts the card has reached its limit...