Upgrade the mobo. You will get newer features and faster DDR3. For the $300 it will cost for RAM, you can get a awesome mobo, 16GB DDR3 and still have money left!! DDR3 is cheap cheap right now (good ole supply and demand).
I'll leave that to an AMD fan, I'm an Intel man myself and not too hot on AMD.
I can give this advice. Take inventory of ALL you peripherals. Then go to newegg, start sorting things by price, AMD and look at all the boards that have what you need or might want in the near future, like USB3.0 support, SLi/Xfire etc. I'm telling you very few boards will have everything you NEED and WANT simultaneously.
ie - I wanted an ASUS Z68 board when I bought my Gigabyte. I owned 3 prior ASUS and trusted them. But none of the ASUS had USB 3.0, PCI-e 3 AND PS2 keyboard port (I have an older Logitech wireless KB/mouse that I refuse to get rid of til it dies on it's own!!).
After all the comparing, printing pages of comparisons, marking off crap I didn't need (like tri-Crossfire), highlighting what I wanted and needed, it came down to the GA-Z68XP-UD3P and one other.
The other had eSATA, no firewire, mine has firewire, no eSATA or something trivial like that. I figured I'd need firewire before eSATA, so the winner finally came forth.
Granted, it took close to a week to wade through all my choices, but I'm just that AR if I'm going to spend money on an upgrade, I only want what I want
Usage for the machine? (sorry if already stated, I skimmed.)
The issue you have is getting the 4GBDDR2 sticks. Honestly I'd just get 4x2GB and call it good. 8GBs is still plenty, not sure why you are going for 16GB. Just buy two of these for $100 and keep using your current build.
I would prefer buying a newerboard since it would keep me more future proof . And I like to do alot of multitasking and editing like with photoshop and other media software so I would like to utilize as much ram as a i can.