the hd 6790 will be more powerful then both cards but it uses that much power also it would be better to get an hd 6770 instead of the two gtx 550ti or hd 6790 whether its 128bit bus or 256bit for the price point hd 6770 will performs better then that two.
More bandwidth is always better. Even though the pixel fillrate of the 6770 is "higher", that's the theoretical limit. Actual output depends on clock rate, number of ROP's, and BANDWIDTH. Not to mention you can overclock the 6790 by a few Mhz and get it to be a 6770 with double the bandwidth.
Higher bandwidth helps at higher resolutions and in applying Anti-Aliasing. I think its very worthwhile to have more bandwidth. It's role isn't as big as shaders, TMU's, and ROP's, but if you don't have high bandwidth, those ROP's can be choked of bandwidth.