GeForce 8800 GTS offers an absolutely huge overclock thanks to how generous nVidia and its partners have been with the core. Generally speaking, the overclock brings it pretty close to the performance of a stock clocked GeForce 8800 GTX. With that said though, the GTX doesn't have any problems with overclocking either giving it the ability to go up and beyond what it was doing at default.
If you were thinking about getting a GTX and have thought, "Oh maybe I will get a GTS instead, due to the overclock on offer...", if you can afford the GTX, we would recommend that you continue with that and purchase it because it will be nice to have this kind of performance without having to overclock your graphics card.
One of the biggest things that you have to take note of is the frame rates you're getting with these overclocked cards. A GTX at default speeds with AA, AF and HDR on when running at 1920 x 1200 is getting an average of 104 FPS and when overclocked 114FPS. Is it worth the mucking around for an extra 10FPS? Yes, it is roughly 10% faster but you're honestly not going to see a real-world difference in game and all you're doing is increasing the amount of work your GPU is doing. Heck, you could probably even underclock the GTX if you wanted... but that's just silly.
You have to understand our point though - move away from 3DMark and move away from the FPS difference being 150FPS average and 175FPS average and think, is it really worth even contemplating overclocking your graphics card? It's pretty safe you will come to the assumption that it's not but others will want to overclock anyway.
Yes, overclocking can be done with ease and you do get a nice speed bump on the GTS - though honestly, you have to decide if it's even worth it. If GTS is all you can afford and you want to bump the AA up another level but it runs a little too slow, then do some overclocking. Although, unless you're gaming at levels of 1920 x 1200 and above, stock performance is clearly going to be enough for the time being it on the majority of the latest games.
Let's wait for some new games to be released, such as Crysis or UT2007, and then you'll probably want to overclock as far as you can as these games will be able to stress out the GPU more than the current batch of games.