I'm using a rig that's 3 or 4 years old and trying to play newer games such as starcraft II beta and supreme commander II. In order to run either of these games I have to use the lowest settings and even then the loading time is excessive particularly in sup com 2. I'm trying to figure out what I want to upgrade first or if I need to do a multi-part upgrade. I don't have the $$ at the moment to build an entirely new system.
My specs -
2 GB RAM
4200+ x2 AMD processor 939
Nvidia 7800 GT
Asus a8n-e motherboard
Well for starters, you should probably go to a 9600 GT or maybe the rebranded equivalent of that. When you have the money though, a change in RAM or maybe just a whole new chipset+cpu would benefit you the most. I would focus on replacing that 7800GT first though.
Yes a cheap 9600gt or hd4670 from e-bay and another 2gb ram and you will notice a mega improvement, mind you I would not expect miracles, but it will turn into a working gaming rig as opposed to a slide show pc!
Also Supreem commander requires a decent Processor but I have played it on my Brother in Laws skt 939 4200+x2 with sli 7900gt 1280x1024 full quality no aa and it was smooth above 25fps, so yes, 4670 and 4gb ram will play it at 1280x1024!
Wow. Your system is pretty close to what I had (I have an ATI Radeon HD3870 instead of a 7800GT, although I DID have that one before the 3870).
At this point, you're only recourse is to either suffer through what you have, upgrade to a 9600GT or wait until you do have the money to upgrade.
I myself just upgraded my 4200+ X2 to an AMD Phenom II X3 720 BE, Gigabyte motherboard and 4 gigs of DDR2-800 memory for $250 due to being hampered by what I had (Socket 939, DDR memory). I had to get a new motherboard and memory due to the architecture change (Socket 939/DDR memory to AM2+/DDR2 memory). But, it was worth it!
Getting the most powerful video card for your CURRENT motherboard/CPU combination might possibly help, but only to a point because your CPU will bottle neck the GPU. To really get things to work better, you really need to eventually overhaul the CPU, motherboard and memory.