I have the x3990, i3 2120 , 3.3ghz, 4gb drr3 1333mt/s.
I put the 'Sapphire Radeon HD 6670 DDR55 1GB' low profile in it (£70 from Amazon) and it works great with games, Skyrim, ACIII, Mirrors Edge, Dead Island, The Witcher 2(very demanding, put on medium still looks great), Battlefield 3 at highest graphical settings smoothly!
Theis low profile card has the fan blasting air against the case though so maximum heat I get is about 70c though that isn't very bad.
I didn't upgrade psu as I couldn't find one to fit. Only need to upgrade psu if you want to over clock safely. When I do get one, I will have to cut a hole for the socket so it fits xD
There are a few better low profile cards that will fit but I only needed this one as I don't go mad on games.
Hope this helps peoples!