First things, never use any drivers beyond version 56.72 with that card.
Trust me, I'm using a V9980 Ultra myself. Either the ASUS driver or nVidia foreware driver will work- 56.55 and 56.72 offer the best performance. I've tried ALL drivers upwards of those versions with each newer driver only LOWERING performance. So, the lesson is with this card STAY AWAY FROM NEWER DRIVERS. The version I recommend is 56.72, but 56.55 works nearly as well.
Make sure you uninstall the current driver before installing the "replacement" driver. After you successfully install either of the two driver versions I mentioned, then you can either use PowerStrip or Coolbits2.0 for OC'ing. You can use the ASUS provided SmartDoctor- but it doesn't offer much.
Okay, my V9980 Ultra is currently set at 515MHz core/ 980MHz mem. I'm using stock cooling PLUS a cyclone blower to exhuast the hot air out the back of my case. In the mainboard bios (I'm using the ASUS K8V SE Dlx) I have the AGP voltage up to 1.7volts. At my oc the system is unstable with only 1.5volts.
Before you try ANY oc, make sure you have proper ventilation in your case. If not you very well may fry your card. In the temperature section under the Advanced section of your Display Properties, it say the core slow down threshold is 140*C, and that is default. Whether it is true or not I'm not sure. If my card gets towards 60*C then I notice a slowdown in games, so I figure it probably isn't at my OC.
I get around 64003DMark points in 03 with 4xaa/8xaf, vsync enabled, image quality at Ultra High via PowerStip, and a 1024x768x32 resolution. In 3DMark05 (same settings) I'm just shy of 2k 3DMark points.
In 2001SE I get just below 18K3DMark 01 points (with the same settings). I'm using an A64 2800 @ 2.16GHz.
Which means you cannot use the normal driver coolbits or rivatuner methods of overclocking !
So far I've had no problem with either the ASUS drivers or nVidia forceware drivers. The only problem with the ASUS drivers is they seem to cause a "minor" conflict with ASUS PcProbe- that is all. I've used both Coolbits and PowerStrip successfully.