That's correct, the GTX isn't able to use the software overclocking, it must be done in the BIOS. You can use the overclocking tool to test higher frequencies, but they won't stick (as you discovered). You must manually program the clocks using the BIOS. 425 seems a bit low... I'm able to get my core up to 465 and still be 100% stable.
5/11/06 at 7:17pm