DDR2 at 800Mhz is sufficient for the next couple years. It is cheaper to merely add more RAM than worry about have DDR3. If you choose to upgrade later, start with a mobo that has potential for an even larger processor and buy the cheapest one that will run with it. Then, upgrade the RAM and much later upgrade the processor.
Hm, that makes sense.
But can I mix my 1-gig sticks with 2-gig sticks? Like I have 2gigs of ram right now, and they are split between two sticks. And if I buy another 4 gigs of ram, which are also split between two sticks, can I use them together? They are both same speed, of course (DDR2 800).