Well in your case probably either would be OK.
The question will be price and availability near you.
Pauldh posted a <A HREF="http://www.z-buy.com/product.asp?item=VG-M8894070" target="_new">link to a pretty cheap G4ti</A>, and that would be hard to beat at that price!
The R9550 would offer slightly more features in games, especially HL2, but by the same token the GF4ti might get better raw framerates whith noAA/AF.
Here's a good list that has pretty much every combination of cards out there;
<A HREF="http://www.digit-life.com/articles2/over2k4/index.html" target="_new">http://www.digit-life.com/articles2/over2k4/index.html</A>
Unfortunately the testing was done before HL2 and doesn't have Battlefield in it. As you can see some new games Like FartCry will stress out these cards even at 800x600. Notice in that example that the R9550 is in the PS2.0 group with all the shiny features, and the GF4ti is in the 1.1 group. The G4ti gets slightly better framerates though.
Really it's a tough choice. The R9550 will give you some nice new visuals, but for the price listed earlier the GF4ti is a good bargain.
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