Agree with notty22, if you upgrade your graphics card, make it a jump of several tiers, otherwise you will be disappointed.
In this case, a GTX470 is not that big of a jump over a 5770.
Two GTX460's scale very well, and are close to a GTX 480 if you get the 1gb versions which are more expensive than what you linked.
If you can afford it, I suggest you market your 5770 instead, and get a gtx480.
Look at some benchmarks of the games you play and the resolution and settings you would like to use. In some, nvidia will be better than ati, and vice versa. Also, some games will not scale as well with multiple cards.
A single gpu GTX480 is the safest bet for a good jump across the board.
If you can wait a bit, ati is supposed to launch improved models, and nvidia will be forced to respond.
I assume that your gaming resolution is at least 1920 x 1080 which is where a strong graphics card need begins.