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Really not a good time to be buying a GPU...58x0 out of stock, 260GTX only card worth spending money on from the nVidia range and that doesn't really have that much future left in it once DX11 really kicks off.
5770 would be an ok upgrade from an 8800GTX... but I'm assuming his board either doesn't support crossfire or if it does have a 2nd PCI-e slot it only runs at 4x... so he couldn't cf a couple of 5770 :S
Problem with the 5770 performance is quite varied... at times its similiar to a 260GTX, in a very few cases it can beat it, but quite often it drags moderatly behind.
+ 1yeah a gtx 260 would be ideal for you and its faster than the ati 4870 1gb plus it can do physX
- 1Yeah go for the HD4870 1GB is cheaper and faster than the GTX260.