You could go with a 760G motherboard.Basically the 760G based motherboards have the improved SB710 southbridge over the 780G motherboards which use the SB700.However the 760G motherboards have inferior on-board graphics.
The Asus M3A76-CM is only around £50:
http://www.scan.co.uk/TodayOnly/Index.aspx
The XFX HD4870 1gb is around £132 too:
http://www.scan.co.uk/TodayOnly/Index.aspx
The XXX version is the same card as the standard XFX card but is clocked higher.