I've got the same thing, an NVIDIA GeForce2 GTS. Here's some advice:
Don't bother trying to overclock the core. It just isn't going to work.
Instead, overclock the memory. There's no way that it is going to reach 460, so don't even try ... you'll just fry the card. Push the memory clock speed up in increments of 5 MHz. Reboot the computer each time.
Now ... check the card to see if it is stable. I recommend that you use a "hot" game, or a demo like Quake 3. Look for artifacting, such as strange rainbow-like colors, warped or missing pixels on characters, and especially, something that looks like snow. If you see snow ... shut down the game, and lower the memory speed.
I also suggest that you download and install 3DMark 2000, from MadOnion.com. This program will intensively stress your system ... specifically, the video card. If you've got a problem, this program will lock up quickly, and if you have managed to overclock the card (and still have a stable system) you'll be able to see the results of your overclocking.
Be sure that you have the latest drivers, and that your AGP aperture size in the BIOS is correct. If your system can take it, enable fast-writes for the memory. You'll gain an extra 200 FPS just from that alone.
There is an extensive article on overclocking the GTS at AnandTech. I found it extremely helpful:
http://www2.anandtech.com/showdoc.html?i=1246&p=5
With a decent cooling solution, I was able to raise my memory clock to 393MHz. You should be able to do something similar. The card is stable at this speed ... I've had it overclocked for 4 months.
Good Luck ... Toejam31