sli and crossfire are only viable options when minimum framerates can be kept well above 30fps because of microstuttering.
It is almost always worth it to spend extra cash on gpus up to the 670 level around $420.
For the average gamer doing sub-1080p resolutions on mid-level settings a radeon 7770 is a phenomenal option.
For the ambitious gamer doing 1080p at mid-settings going up to the gtx 560 ti or a gtx 480 at the $220 level makes the most sense.
For ultra settings on 1080p at constantly smooth framerates, the 7970, 7950, gtx 670, and the soon-to-be released gtx 660ti are the best options.