yea the 7800GT is like 450/1050 (bfg) and 400/1000 for the stock one.
anyways on topic i have run 18 loops of 3dmark05 with my gigabyte 6600GT at 579/1303 and the only reason i stopped was cos i felt like some world of warcraft (or world of warcrack as it should be known)
you should be able to get atleast a 100mhz oc on the ram i would think but is it one of the agp ones that was 900mhz stock? also it might depend of which ram chips it has beacause for instance my 6600GT uses the 2ns ram chips that overclock further but urs might use the 1.6ns chips that are faster but don't oc as far