Go for a ddr3 based set-up. You’ll definitely need more than 2gb in the future, and ddr2 won’t be available easily a few years from now.
I agree with labrodor100, get a dual core processor.
If you don’t want to spend too much, consider getting a second hand Athlon X2. Lots of people are moving to quad cores, so you should be able to find one fairly easily. And I’ve never know a processor to die, so it’s pretty safe getting one second hand.