@ Nocteratus: I have to disagree, I went from a 8800GTS 640 (G80) to a HD4870 and noticed a useful improvement, particularly with AA, a known weak point on the G80.
All of which is a little moot, Socrampy has a G92 card anyway, Nvidia never built G80 8800 with 512 Mb of VRAM, just 320 and 640Mb.
Despite my technical nitpicking, the jump to a GTX275 will be quite large; Given a little overclock it can easily stay within reach of the GTX285 and can out run the GTX280.