Best PC graphics card

I don't know much about computers, but I am interested in pc gaming and would like to have a better experience. I've already upgraded my ram to four gigs and the next thing on my list is graphics cards. I'd like to spend under about 150 for one. What is the best brand and model? I've been hearing a lot about navidia and ATI Radeon whatever that is.

by the way, I'm using XP if that would affect anything.

And I'm using a Dell Dimension E510

The ATI HD 4770 looks like it is a pretty good budget card. However, your computer has a single core processor and it will hold you back in many games (You can play most of them, however, on lower settings). If you want to get into maxing out graphics on high resolution games, then you will need a new rig all together.