Not really. You may lose like 5-10% performance. That said, I do not recommend it. CrossFire HD 7770 is already weak in general. If I were you, I'd try to sell the HD 7770 and upgrade to a better card. The CrossFire is weaker than a single GTX 660 or a single HD 7870,
Here are some of the games that I play: Dirt 1,2,3 and showdown bf3 mw3 Crysis 2 and starcraft 2. And in these games the crossfire set up cripples most of the higher end cards and is only 5 FPS away from the 7970 in dirt 3 at ultra in 1080p. By the way I'm 14 so I'm not some enthusiast or anything.