I'm running Vista Ultimate 64 on a p5q deluxe with 8gb of Gskill. It's running 2X4. It installed and runs flawlessly, I'm not sure if you still have to install vista with just one 2gb stick and then plug in the rest after the install completes but I did it anyway. All I did was make sure the memory I got was on the qvl I downloaded from the asus global site before I purchased the parts. It also helps that the memory runs at 1.8v, timings aren't perfect but for around $60 a stick I can't really complain. There's that mushkin kit everyone on this site recommends (its on newegg for around $64/2gb stick, 1.8v also
http://www.newegg.com/Product/Product.aspx?Item=N82E16820146731) but it wasn't on the p5q deluxe qvl so I didn't wan't to risk it. I'm sure it would've been fine since it runs at 1.8v, the timings are a little bit better than the ram I chose but I don't think I would be able to tell the difference. Doing a decent amount of multi-tasking (MMORPG, Music, firefox with a few tabs open) the highest I've ever seen memory utilization is at 30%.
Edit. I forgot to add all the mentioned memory runs at 800mhz. For the p5q deluxe there was much less compatible memory running at higher speeds. I didn't need higher personally but I think asus still has some compatibility issues even after bios updates.