Hey Shaplus,
The actual difference in DDR2 Vs DDR3 is not much, see this review by
tweaktown. You see there is a difference but not so much that you can drool over + the cost factor involved is prohibitive. DDR2 will be certainly here at a decent price for another 2 years or so, then it will get expensive later on due to DDR3 becoming more mainstream and affordable.
I don't know why you are bothering about 785G mobo's anyway. It's got a better onboard GPU but you aren't gonna be using it, so why bother. I bought an Gigabyte Gigabyte GA-MA78G-US2H (780G) chipset mobo because....
1. I really don't need a better featured board. The features on my current mobo are more than enough.
2. I'm not going to overclock inspite of my CPU having that capability.
3. I went for Micro ATX simply because a full blown ATX would just be waste of money for my use.
4. Yes I do like gaming, therefore have bought an NVIDIA GTX260 which is running fine, but needed to buy a L-shaped SATA connections as the card wouldn't fit. Consider that before buying a GPU, check the length of it out. The damn 260GTX is a looonnnggg card...simply ridiculous.
5. Don't forget that normal ATX casing like iball or VIP brands are not sufficient for long cards. AM using Coolermaster Elite 330 which lets me fit long cards in it.
6. I've seen that every 2 years or so I make a purchase or upgrade as I see hiccups with older computer components coming about, due to dust etc. I have a lot of dust at my place. My house is bang infront of the highway. Therefore I know that I will look into most probably Intel config as they are ruling the roost right now and will definitely get cheaper later 2 years down the line, unless of course AMD should come out with some mad beasts that will steal the Intel crown, it all remains to be seen.
Hope this thought process helps in purchasing.