You can always OC the 4850 or 9800GTX/GTS250 as well...
The 4850 will OC reasonably but it won't get near a 4870 due to the memory difference (GDDR5 vs GDDR3). On the other hand, a 4830 is a 4850 with a few less shaders and without doubt it has an artificially lowered clock speed. I've seen plenty of 4830's overclock higher than 4850's have.
The 9800gtx (see: gts250) is already the top of the range G90's. Some will OC pretty well, chances are they are the ones already being sold as gts250's however.