I have the following and was thinking of adding 2 x 2GB of memory to make a total of 8GB, if anyone thought it made sense
AMD 720 X3 Black Edition which freezes my MB when I tried to unlock or overclock
Biostar TA790GX 128M
4GM RAM - G.Skill F2-6400CL4D-4GBHK DDR II 800 CL 4-4-4-12
2 HD's - 1 WD 640GM and a 1TB
Corsair 650 power supply
Apparently that memory is no longer produced. I'm hoping just to plug in the new modules and have them work without having to manually set anything.
DDR2 memory is expensive. I would suggest you either buy an 8GB kit (between $100 and $150) but you can try buying a 2x2GB kit to supplement your current one instead of replacing... However I can't recommend that. Mixing RAM kits is not guaranteed to work, especially if you don't get a kit that is identical to the current one.
DDR2 pricing is unlikely to ever go down so you might be better off holding off on upgrading and replacing the motherboard/CPU/RAM with new parts sometime next year, preferably after Ivy-Bridge's launch supposed to happen in mid 2012.
I think I'll follow your advice and wait for a while. This motherboard doesn't support AMD X6 CPU's so that would mean a new MB, plus a new CPU, plus new memory. Sounds like I'd be better off going the Intel route at some time in the future.
Next year Intel will start selling Ivy-Bridge. Ivy has much greater performance per watt and considerably better performance per clock than Sandy bridge. Considering that those are Sandy's strong points right now compared to all other common CPU architectures, Ivy is nothing short of spectacular looking so far.
The top Ivy i5 and i7 CPUs are supposed to be 10-15% faster than Sandy's top i5 and i7 yet they will only have 77w TDPs compared to Sandy's already low 95w TDPs.