Having had a job at a certain... evil retail store that builds and sells machines (As well as every other imaginable electronic product), I can saftely say that your current rig needs RAM.
Joint Ops would fly a lot better if you upgraded that. Problem being, upgrading SDRAM would cost about the same as getting a Motherboard and CPU.
CPU's can easily be overclocked right out the box with no "harm." You can buy a P4 2.4 800FSB and OC it to 3+GHZ no sweat. The same could be said fro the AMD 2500+, which overclocks right out of the box to 3200+
Motherboards are more a personal preference, as some people swear by ABIT only, or ASUS only, some people are DFI fiends. You get what you pay for in a motherboard.
RAM you also get what you pay for. Get DDR that is going to run on your bus speed, do not bottleneck yourself getting cheaper slower ram. I perfer Corsair XMS, Hyper X is good as well. Get 1GB, (2 512mb sticks) and run them in dual channel mode. Getting a pair of XMS won't cost as much as you think.
Hard drive speed isn't really as important as you think. Nor is size. I perfer to run extremely fast, small drives, either SATA or SCSI. Right now I have 2 40GB Raptor SATA drives.
Video card is also important. That all depends if your going AGP or PCI express, so that would depend on motherboard choice.
People are divided between nVidia and ATI, I've had shitty cards from both, and good cards from both.
Bottom line- You need more RAM, lots of it.