8000 MB = 8GB.
But 3 GB is also ok, usually most games require at most 2 GB to run, and games should be some of the most requiring programs, so...
But Universe Sandbox is a simulator, and the most important in simulators is the processor, becuase that's the one that calculates everything (the positions of stars etc.). Your computer should be fine at Universe Sandbox.
I paid something equal to 359 USD for my graphics card. It causes some problems, but when it works it's fantastic. I have no problems with graphics on the games I've installed so far, I can run them at maximum graphics. I guess I just have to wait until there are some updates for the drivers so it will be more stable.
But everything is better than my old computer.