A bit of a loaded question this one as it all depends on your choice of CPU and how much RAM you are intending to put in the system.
I have just got hold of a 16GB kit of Corsair 1600MHz RAM in anticipation of the new AM3+ Fusion FX processors (when they come along) but, with my current 1090T Processor and GA-990FXA-UD5 motherboard I can only run this at 1333MHz. The system refuses to boot if I try to increase the RAM to its default speed and keep all four slots populated, even if I up the voltage. Not a big issue at the moment but, I will not be pleased if it still won't work when I drop a new CPU in.
If you are going for 8GB (2X4GB sticks) along with a new AM3+ CPU then you should look for RAM at 1600MHz or 1866MHz as the new processors will support these. But remember, the current AM3 CPU's really don't like RAM running above 1333MHz (although it is possible to run RAM faster) and if this is your intention then I would suggest sticking with 1600MHz RAM and again a 2X4GB set rather than 4X2GB's as the processors seem to prefer it if you run with a single stick on each channel.