Please be aware that currently there is not much of a speed advantage between DDR2 and DDR3.  Since the DDR3 technology is still 'new', it still costs much more than DDR2.

With DDR2, the newer motherboards support up to 16Gb of RAM, and DDR2 is now available in 4Gb sims (sticks)... so 4 x 4Gb = 16Gb of ram in a system.  To utilize that, you need to run either "XP64" or "Vista Home Premium 64" or "Vista Ultimate".  "Vista Home 64" is limited to 8Gb (MS rules).  Not sure about "Vista Bussiness 64" but I'd suspect it's good for up to 16Gb.