Kingston Hyper X PC3500 or PC 3700?

I'm looking at getting a match set of 256's in either PC3500 or PC3700.

Which is better for me to go with?

Thanks for your time

Check the timings on both. Both sticks might have the same exact memory chips with different timings, where the PC3700 is at slower. Also compare the price and do some searches in the forums to see if people have posted experience with both modules.

This is a common practice with every memory manufacturer. Doing a little research will keep you from (unreasonably) lining someone else's pockets with your cash .