The 6990 has 2 cores in its silicon, whereas the 290x has one.
If you plan on buying the 290x, replacing the cooler for a better one and overclocking then you might see better fps on the 290x.
Also, having a single card has its benefits, such as no 'microstuttering' (frame times are much more consistent) and future upgradability. However, seeing as AMD fixed most of the microstuttering in crossfire, if you're happy with your card I would suggest holding on to it for now.
#1: Phenom II x4 B55 @ 3.8ghz | Gigabyte GA-MA790FXT-UD5P | Sapphire hd4870 512mb | G.Skill Sniper 2x4gb | Antec 900 | Corsair TX750 | Samsung P2350 1920x1080
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