I'm using 77.77 on my 6600GT and they work just fine for WinXP.
For Windows 98SE I use 77.72 and again they're pretty flawless as far as I've tested them!

@F2bnp: do you have problems running older games using 6800 on Win98SE?

I don't have a 6800, only a 6600GT. Never had any problems with 77.72, but then again I haven't tested a ton of games either. Most of what I want to play on that machine works fine under Windows XP.
I think Blood 2 gave me some problems with this card, regardless what OS I tried it in, but then again it's quite a buggy game!
Shogo worked flawlessly!

I only tried with the latest 81xx drivers from nvidia, can't say that I see more fps than my 4200ti/64MB on my p4/win98se machine... and I only gained ~+4.000 marks in 3Dmark2001se... (it scored ~15.150 IIRC)
I would expect more from a 6800GT with 256MB DDR3 vram... maybe it was wasted money for a win98se/agp machine... payed 29e for it but needs a silent cooler too...

as a sidenote I could not play BloodII correctly with either the 4200ti or the 6800 and various drivers installed... some tricks can get you going but all have some quirks.
IIRC leileilol said this in a nvidia "feature" about frames sync or something, ATI cards work OK ...
I reverted using the v2/sli setup though for this game, so not all is lost - just some fps and resolution 🤣