Hey,
The 4X will reduce the scaling from the second card, but not so much that I wouldn't add it. There are a fair number of articles(some of them here) in regards to the diffrence from 4X, 8X, and 16X. I'm often surpised by how small the diffrence is between them. 2 5770's are a cheep way to get great performance, I just replaced mine with a single card solution, but I was very happy with the performance I was getting.