Hiya pal
From personal experience I would say no ....
I was in the same boat, but I had the advantage of running 2 x 5770's in crossfire and because of my job I happened to be able to test a single 5850 aswell, from personal benchmarks I found that a single 5850 out performed 2x 5770's in crossfire quite considerably.
So personally I would advise that you leave you MB as is and purchase a single (larger than you have card) you could always sell your 5770 and recoup some of your cash towards your new 1