I just replaced my Geforce MX card with the Geforce 4Ti from the Apple Store.
Apple uses the Ti4600 card, I've read this in several places but where escapes me right now.
The only store I've found other than the Apple Store that carries the card and has it in stock is BH Photo & Video, but their price is even higher than Apples!, $434.95.
I was hoping that the ATI 9700 would be released soon but so far there is no info. on when and if (though I'm sure they eventually will) release it for the Mac
If your running OS X 10.2 it's just plug and play, I installed it without installing the drivers from the C.D, its running perfectly with the drivers bult into OS X 10.2.
With OS X 10.1 you might have to install the drivers, I remeber some people had issues.
By the way, the Internet Apple Store says there is a 3-4 week wait but I found several Apple retail stores that have it in stock.
Hope this helps.