Depends on resolution and games, if it's a new pc wont you be using vista or windows 7 in which case a 5770 brings the benefit of DX11. Not too sure on the numbers but i'd think a 5770 would be stronger in most cases over a dual 4650
Also 4770 ~100 bucks in the US
2 4650 is ~120 bucks in the US
5770 is like ~170 bucks in the US
It's the most expensive by far
If you have no need for dx11 i would think the best thing for you would be a 4870 512mb edition for about 130 bucks US
No. Two 4650s in CF preform about as well as a Radeon 4770 and a little below a Radeon 4850. Considering that a single 4850 can be had for around $100, I would go with that. A 5750 or 5770 would be a better option though as they are both faster still and have DX11.
I was looking into the 5770 and 4770, and I liked both, but my only problem was that they were both 128-bit. I'm not sure if it would be a wise decision to chose either of those two. However, I would like to know the difference between 128-bit and 256-bit.