i would agree if you already have a 5770 why drop it off, when you can save some cash and get another 5770 for crossfire, even though i'm pretty much in love with the 460, if you've already got a 5770 which is pretty much as good price performance as the 460, but for 150 bucks, you can get performance better then one 460, and honestly it should last you at least, until the next gpu generation.
Unless of course you really need a high power multi-monitor display or 3d. if your gaming at 1920x1080 or less, don't bother on anything besides another 5770 to CF them up