Because your computer is older it requires an AGP type graphics card. Most cards today are the newer PCI2x16 kind--so be careful when purchasing.
Personally I would recommend the ATI 4650 for about $70http://www.newegg.com/Product/Product.aspx?Item=N82E16814125281
It's more powerful than you need-but might help your aging computer continue to be useful a little longer.
If you wanted to spend a little less the ATI radeon 3650 or 3450 would be cheaper--and still give you enough of a performance boost for some moderate gaming. (higher numbers are typically better among ATI products)
The manufacturer shouldn't make a great deal of difference--especially since you just want a low end card for an older computer.