Wow, 3-6 fps, that's kind of epic. I'd suggest a new computer.
Lots of money is required there. Right click My Computer, then go to Properties. Under Computer, tell me what it says in there. Should be your processor and ram. But for the time being, get a better graphics card, it will help lots, only if you wish to be able to use 2 computers. Unless you really want a new computer. Btw, its not lagging, its just only showing 15 frames a second. If its over 50 or something, our eyes don't notice it at all.