Give Mandriva a try
2008.0 RC1 is out on the mirrors, and RC2 is to come this week (final possibly in the end of the month). Your card will work out-of-the-box (i have a 8800GTS320Mo which is correctly supported by nvidia100.x drivers, available in Mandriva already during the installation).
Prefer the network install in order to get the most of your system (removable media cannot handle all the packages available except if you use 3/4 DVDs ...).
I've seen reports that the XFi works also, but i have only a SBLive! (which works fine, btw) and i don't suffer the AHCI issue you're mentionning (i have an Asus p5-E+ motherboard).
Plus : you can use the 64bits edition, runs as fast as 32 bits and while needed, the software layer for 32 bits applications is present (like flashplayer for instance).
Feel free to ask for more help.