sonjay - 2:41 pm on Feb 9, 2010 (gmt 0)

I usually use something at least 20 pixels or so wide -- not because of performance considerations, but because I find it easier to work with in Photoshop, and easier to troubleshoot if the background image isn't showing up, or if it looks wonky.

Can't say I've ever noticed any performance problem either way, though.