If I were you, I wouldn't upgrade without just going AM3.
Wait and save your money until you can afford new RAM + new motherboard.
For now, you could just buy an AM3 CPU and wait to do a larger upgrade later.
Upgrading your motherboard without moving to DDR3 and AM3 doesn't seem worth it to me.
And you'd be stuck with DDR2/AM2+ using an AM3 CPU until you finally did upgrade in the future...
Your AM3 CPU would be bottlenecked for even longer if you upgrade the motherboard without going AM3.
[ Just to clarify what I'm talking about... ]
AM2+/AM3 motherboards only take DDR2 RAM
AM3 motherboards only take DDR3 RAM
AM3 CPU's can work in either motherboard though, because AM3 CPUs have both a DDR2 memory controller and a DDR3 memory controller.
Though you could consider running an AM3 CPU with DDR2 RAM as a memory bottleneck.
Luckily I just found this article...
So I guess Phenom II X6 will be backwards compatible...
So AM2+ will survive for a while longer I suppose lol.