I Need Video Card Advice

[theRepublic]

I have an old computer that I bought about 6 years ago for college.  It still runs great, but I haven't been playing anything other than flash games on it.  I think Star Wars: Empire at War is the last real computer game that I have tried running on this thing with success.  I want to stick a video card in it, but I am not sure what would be worth it and what would be overkill for a computer this old.

Here is the computer: http://support.gateway.com/s/PC/R/5022/5022nv.shtml

Quick link to details: http://support.gateway.com/s/PC/R/5022/5022sp3.shtml

The only upgrade from the original parts is that it now has four 1 GB sticks of memory instead of the original two sticks of 512 MB memory.

The current monitor on this computer is 17" and 1280 x 1024.  It is possible I will upgrade it at some point.

I am looking to be able to play Portal, which just crashes when the first portal opens right now.  I also have a friend that wants me to try out League of Legends with him, so I need to be able to play that too.

Any suggestions on what to put in this computer?