Buy more GB. as RAM speed makes very little system performance but more RAM allows you to open more apps. concurrently or use larger apps like CAD, video editing, etc.
NEITHER increased RAM speed nor RAM quantity will improve your PC system speed substantially however. 8 GB. is what most people use with Windoze as it's just right in quantity and currently in price. For most PCs DDR3 1600 Mhz. RAM is good though the new AMD Bulldozer uses 1866 Mhz. by default.