Building your own > Monarch/CyberPowerSystems/Falcon Northwest/those kinds > Dell, HP, Compaq...
You really SHOULD get a whole new PC. Chances are, if you get a new videocard, that there is something else in your PC which will slow it down and then the videocard will be pretty much useless. I know the kind of situation you're in right now though. You should have your Mom/Parents take a look at this thread.
A good component in a bad computer = bad computer. Your parts all have to be on the same level. I know I keep saying this over and over, but think of it this way; a level 12 part, in a level 5 computer will only come out to be a level 5 computer (There are no such things as computer levels, I'm just making it up for the sake of showing you).
So its best to built your own. If you can't (its not that hard), then go with Monarch Computer Systems or some maker like that. With those, you just need to know what parts to choose (and we can help with that).