They used to be horrible, they've gotten better since ~ a year after the takeover by AMD.
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Is a GTX 275 good enough?
More than.
As I mentioned elsewhere, I'm using an 8800GTX with no complaints. I FPS-limit the slaves when they're not focused, but even if I had infinite GPU power I would do that to save stress/power, and I haven't tested it without. During an RMA, I've also used a 7800GTX and an 8800GT.
Anything midrange or better ---> probably fine. GTX 275 = very powerful --> easily more than enough.
I was using a 2GB (Sapphire) Radeon 4870X2.
Pretty happy with it, up until I upgraded to the i7 920, 12GB ram etc.
Otherwise same system.
But then slave windows started to freeze on me.
Going to the 275 GTX, none of the slaves are freezing.
It is basically the same power as a 1GB 4870.
DDR3 instead of DDR5, but plays wow as smoothly on the same settings.
Now that they're not freezing, I've upped the view distance on slaves to maximum.
And spell details maximum across the board too.
More then I was getting, on the 4870X2 (with settings down for less freezing).
Same smoothness as I had before on the other card, with settings the same.
Or short version.
Yes, it's overkill for wow.