they only make 128 mb ones in PCI for the 5700 at least thats all i saw on newegg but it should do well and it looks like a decent graphics card and should play games at least on low settings heres a link to one
those cards will run CS easily im sure.
in my opinion your tower is outdated and depending on your budget you may want to get a new box (insides and all).
what games though? all those cards will run games... depending on what quality.