If that's the right one, it doesn't support DDR3 memory. It supports up to 1066mhz DDR2 memory. If you were to upgrade to that speed, you would notice very little increase in performance. Probably only noticeable in benchmarks and not in real-life.
STOP The P35-DS3x line of boards are known to have issues with RAM above DDR2 800 (ie DDR2 1066). Get a good quality set of DDR2 800 and OC it. For more information see: GUIDE: Overclocking On P35-DS3L
DDR2 and DDR3 are NOT backward compatible If you are not OCing stick with the DDR2 667 RAM.