I know for one that Deus Ex runs horribly on my WinXP system with the Geforce 3 Ti200. Lots of lag despite being a 2000 game.
Proof that indeed no driver can be effective on 99% of games.
I doubt ATi has drivers which support only 80%, it's more like they can play the majority if not all, but some may have glitches. Currently I don't see frequent glitch reports for NEW GAMES or recent ones.
One thing I always worry:
While the R9500PRO is probably the next Ti200-Ti4200 P/P card, I feel like buying them, despite having future proof capabilities will not help! That's how I always felt, as you don't even know how the next generation of games will push. I'm even afraid the new games will push the R9700PRO to low levels of gameplay that it can play at 60FPS, albeit many drops to sub 30FPS. This is one of the reasons I get afraid of the new cards which claim future proof support but in fact you never know just how far the games are pushy.
Aquanox had nearly full DirectX 8 utilization, yet it ran at about 50FPS on the Ti500 which also would drop to 25FPS in intensive action, all the while being the highest end card back then, if not the standard GF3.
As you can see, it's an unsure market, that even the best card could suddenly lose to games. But we'll see how proof comes, I just want MINIMUM FPS reports more frequently like UT2003 does.
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*You can do anything you set your mind to man. -Eminem