Switch from Nvidia To ATi?

I have an older computer: windows xp Pro SP3, 2 40gb Hard Drives, 1.25GB ram, and a Nvidia Geforce 6200 256mb PCi card. I bought the Card a couple months ago, and get no better frame rates than my old 2x AGP Geforce 2, theyre actually worse, pry because pci is slower. My friend has an ATI Radeon 9600Pro, I am tempted to switch to ATI and get this card a href="http://www.newegg.com/Product/Product.aspx?Item=N82E16814129126" rel="nofollow"http://www.newegg.com/Product/Product.as…/a or something like it. Is this a good upgrade? my motherboard only supports agp 4x and i cannot get a better comptuer, i will be playing games like half life 2 ep1, etc. What kind of Frame Rates can i expect, right now i get around 15-20 in Deathmatch, not acceptable...