As for the PSU, Dell has been known to organize the cables in a non-standard way. Comparing if the colours match in the big motherboard connector and switching if needed might help. If the PSU will have a new life in a standard setup, resetting the wiring to original should be the first thing to do.
G210 is a very low end video card. It's much better than any built-in Intel video chip, but still low end. Good thing is it can draw the game correctly, but as you've noticed the frame rate drops easily.
As for the CPU this thread seems to have valid information:
http://en.community.dell.com/what-do-i-buy/for_enterprise/f/4516/t/19001903.aspx. A nice tool to find out the socket and much more is CPU-Z:
http://www.cpuid.com/softwares/cpu-z.html. Knowing the right socket will help to find available processors.
Theoretically
any 32 bit OS including Vista will support 4 gigs of memory. Theoretically, that is. Graphics memory is included in that number, and some 500 megs gets lost to who knows where. Having identical values in similarly coloured slots will enable Dual Channel for better performance. Don't bother getting the 64 bit Vista with only 4 gigs of RAM, you wouldn't notice any difference other than difficulties in finding drivers.
It doesn't really matter whether the pci-e is version 1 or newer. The video cards are interchangeable within versions and the bottleneck would most likely case be somewhere else.
For cleaning unnecessary programs and services BlackViper has a very good reputation:
http://www.blackviper.com/. Basically anything starting with "Dell" is not an originally needed Windows essential. You're right about the hard disk reinstall package being bloated. Fortunately any OEM Vista DVD will work as long as you can read the installation license on the sticker. Borrowing or even copying one from a friend is totally acceptable, the license code is the private part. I've even managed to get a copy from a local repair shop for the cost of the disk even before I started my business.
PS Vista doesn't fit on a CD, but a DVD will work similarly.
