I agree that you should look into PSU issues and possibly replacing it. Regular case fans and and as long as the heatsink and cpu fan are installed correctly should not have any problems keeping the system in a usable state for at least a decent amount of time, unless there is SIGNIFICANT dust...
You need to make a whole new computer. Most likely you have an AGP slot so the HD4650 will probably not work. I don't know much about ATI cards but I believe that card is a PCIe card.
I think some later P4's had PCIe so I could be wrong, I don't think you mentioned what you have but you should...
I've seen various combo deals on PSU and Graphics Cards. You should upgrade both. 280 watt is not enough to sustain most discrete graphics these days. Also you need to upgrade your RAM, I recommend 4 GB total minimum.
This very much sounds like a thermal problem to me. One thing that most power users seem to overlook is the PSU fan. Sometimes the fan in a PSU will die and you would never notice it. When under heavy stress the PSU can overheat like the scenario you've described.
Also, you should make sure...
You won't only lose speed from the memory clocks, the CAS latencies will be out of sync as well which results in both stability and speed issues. It's highly unrecommended that you run two different types of RAM on the same computer.
When using task manager to set affinity, there are many "CPUs" listed, is there a way to figure out which are the physical and which are the virtual cores?
I am running dual X5680's which have 6 physical cores and 6 virtual cores each. I see 24 CPUs available in the task manager screen.
I...