Hi men, few months ago I used 1066MHz, and now I have 800MHz and the difference isn't too much. If you want that your system run more faster, you must be buy DDR3...but how I said,the difference isn't much. Get the 800MHz and with the rest of the money buy something else.
Whatever OvrClkr said, it's true. I've had a pair of 2GB Corsair rated to run 1066MHz at 2.1V. However, my motherboard did not like it despite it supporting speed up to 1600MHz through OC. In the end, I swapped for a Kingston HyperX 800MHz with tight timings 4-4-4-12 and I am very happy with it. The performance is solid and I'd suggest you do the same instead of going for higher frequency.
The system is designed to run FSB 1600, ram @ 800.
If you plan to raise the FSB go with the 1066 otherwise, 800 is perfect.
Look for 800 MHZ ram with a low CL, something like CL 4-4-4.
Saint19, you are incorrect, ddr3 will not be faster then ddr2 because the timings on ddr3 are loose compared to 800mhz ddr2 with CL4. Furthermore the P5Q davidstone doesn't support DDR3 -> four DIMM sockets that support up to 16GB of non-ECC, un-buffered DDR2 memory at speeds of 667, 800, 1066, and 1200MHz