You wouldn't get much performance boost for 2x460 vs 2x275. Just cooler running, DX11, and a bit better scaling. Maybe if you could sell the 275 for a good price and find a couple 460s for cheap, but as that's not likely I would suggest getting another 275.
@ Tillman32: If this is for the system in your config the point is moot- The GA MA770 UD3 motherboard does not support dual cards.
And what resolution do you use? SLI is likely to be wasted on a 1280x1024 display.
If this is for another system and for a monitor of 1680x105 or above then go for it if the GTX275 is cheap! There's still plenty of life in DX9/10, just be certain the PSU is up to the job, these 'older' cards burn more jiuce than the current generation.
Well, glad that's sorted then
At that sort of resolution I think you might need to look further upmarket, the GTX460/275, even in SLI, is likely to struggle with three large monitors, but I could be wrong- anyone like to comment?
Performance wise the GTX460 just edges the GTX275 out and with better scaling with cooler, quieter operation as added benefits.
If the new MB also supports Crossfire you might want to hold fire until the new HD6xxx series is released and see how they stack up.
Me, I LIKE cool and NEED quiet, so my choice would be the GTX460 and E-bay the GTX275.