Here I think I can simplify it more for you.
Steam running= "some" memory usage. For the litte you have, that means allot.
Steam NOT running= A tiny bit more memory is freed up, and it seems like your computer is running faster because of it.
Bottom line, when you read system requirements for a game, that's JUST FOR THE GAME itself, and nothing else. Steam has its own requirements, Along with Windows! You need to add up all the ram needed to run everything.
Steam requires 512mb
X3ap requires 1gig
Windows XP 64mb
So far thats 1.576 Gigs
AND THATS NOT counting anything else you have running. I.E;
Virus programs
Music programs
any open windows
Web Browser
etc.....
So in the end, you DON'T HAVE ENOUGH memory to run everything. Again, I'M NOT PROTECTING STEAM, just trying to have you understand why your experiencing the problems that you have.
With ram usage that high, with everything running, you must experience allot of crashes, restarts and blue screens.