Try these mate,
ForceWare Windows XP/Media Center Edition (assuming you're on XP)
My 320Mb 8800GTS card got quite a boost from upgrading to the latest drivers.
Also sudden frame dips can indicate Spyway and a whole other host of stuff, like maybe your AV software is doing something... a common problem. If its just at graphically intense moments ignore that and just make sure you are running the latest chipset and
GPU drivers.