I couldn't manage it on my P5Q-D or -E at all, and I tried with 1296+ capable reapers, and a pair of Gskill PI9600's which were good for 1250+. Even the 2 MIIF boards with their very fine GTL controls got me a barely benchable 506x9. I only know some with older Q6600's got stability with 500fsb, but less so on 45nm quads. Maybe I didn't look into the P5Q enough, particularly after hearing it clocks better than the MIIF, but I spent enough time (almost a year) trying with the Asus range and moved on.
All 3 of the EP45 range I now own do 500FSB with ease and good enough cooling, and have benched beyond 540. I admit that a change of mobo would be a bit drastic for that 100-200MHz increase though!

4 dimms won't make it any easier for sure, you might want to try 2 sticks for now and see if that improves things. If it does, maybe try increasing MCH volts with all 4. Try a higher TRFC for 4 sticks too, something like 75-80 to be certain. You could also raise your performance level to ease NB stress.Good luck.