The price of DDR2 isn't going to go much lower. DDR3 is going to be more popular soon, with the new i7 CPU.
It's always a bit harder to populate all 4 memory slots. By getting 2x2GB now you'll avoid that, and you won't have to think about upgrading later. CPU intensive programs usually benefit from more memory. It certainly won't hurt, and it's very inexpensive.