1. Virtual memory is like your comps short term memory.Its can remember things for a small ammount of time and is often cleared by your comp...but some things use much more then most and some settings make your computer use more.The best way to explain it is to imagine you are playing a game(a shooter is the best example).The level you are on now will the in the short term memory.When you leave this one to go to another one the next one is loaded into the memory.Things like blood splatter are saved onto this aswell.512 megs of virtual memory is pretty much the minium for running any new games now but the more the merrier.If possable get or have 2 or more gigs of it as you will notice it in your computers performance.You buy it in sticks which are insered into your comp(the inside on it by opening it up).Some Motherboards can only have so much on board memory so its important to find out aboot things like that before upgrading or anything like that.
2. Im really not sure tbh...
3. same as two im afraid.
I hope i help at least with number one.If anything is unclear or wrong please feel feel to correct meh