If your using stock voltage on the card then 425 is a pretty good OC. I had to bump with the voltage on mine in order to break 400. I think you could probably get a little more out of that card if you flashed the bios with a slightly higher voltage.
I still have the exact same card using the NV5 on it (predecessor to the accerlo) I could get about 420 max myself. It would do 435 but running 3dmark03 it actually yielded lower scores so I backed it down to about 420.
Here is a guide on howto flash the bios for the card. http://www.mvktech.net/content/view/2069/37/
Default voltage on the card should be 1.3V, and I bumped mine up to 1.4V using NiBiTor. http://www.mvktech.net/component/opt...rent,category/
Depending upon the age of the card, you might already have the latest bios. Version: 5.40.02.15.03 I actually grabbed on from their site and changed the voltage and flashed it to my 6800GT without issues. If you load up nibitor you can read your current bios and then dump that one to make a change. If it's up to date it is best to just use what's already on the card since you know it's working. If not then you can get the newest one here. http://www.mvktech.net/component/opt...rent,category/
If you decide it's something you want to try, just give me a PM as I actually have the latest bios for an XFX 6800GT AGP card already changed on my thumb drive from when I flash my card. Just remember that flashing can be a risky business because if it fails the card won't work properly. If you have a PCI video card, you can enable your bios to boot from PCI first then AGP, so if your flash does fail you can boot up the computer with both cards in it and then flash back the old bios to your 6800GT. Always make a backup of what is on your card before you attempt something like this. You can use Nvflash http://www.mvktech.net/component/opt...rent,category/
to change the bios file, but make sure your doing it in DOS and not windows using a boot disk of some kind.
EDIT: One more thing, I want to say that if you have an older driver like a 92.XX and coolbits you can unlock the ability to change the voltage right in the drivers. This way you can test it first to see if it's even worth the hassle of flashing the video card with a new bios. I'm not really sure anymore though cause that was a long time ago.