Legit Reviews spend some more time with their GeForce 9800 GTX+ card. In their original review they found the card was on average 10.4% faster than the Radeon HD 4850 and now they've tested how high they can clock this baby:
The highest settings we could get stable with the GeForce 9800 GTX+ were 855MHz on the core and 2200MHz on the shaders. That is 55MHz higher on the core and 175MHz higher on the shaders than what we could reach on the old 65nm G92 core. For those that like percentages the overclock is roughly 6% higher on the core and 8% more on the shaders, which is significant, but nothing over the top. Usually with die shrinks overclocking performance stays close to the same, even more so when no major architecture changes are made. It should be noted that the memory overclocked better on the GeForce 9800 GTX+ and it seems NVIDIA has moved over to Hynix branded GDDR3 memory ICs. The GDDR3 memory IC's on the new GeForce 9800 GTX+ card were Hynix H5RS5223CFR-N2C, which are rated 1200 MHz (GDDR3-2400) 0.8NS with 1.5V. These are the same memory IC's that were found on the GeForce GTX 280 cards we reviewed last week..
Check it out over here, the reviewer concludes overclocking the GeForce 9800 GTX+ improves gaming performance across the board by 15% on average.