Let me say this, right now you still have the opportunity to sell your 5770 and topup few hundred ringgits for a better card, because once you CF you have to commit to use those 5770's for a long while...so you gotta choose between sticking to the 5770, CF it or trade/topup for a better card...

Why am I mentioning this? because two 5770 is slightly better than 5850 but cost around RM600+RM600 = RM1200... 5850 is only around  (even below) RM1000 and needs a moderate OC'ing to match those CF'ed 5770s...
Keep in mind CF can sometimes micro-stutters in certain games...

And it's easier to sell one great card then two good cards...

Remember, you're 5770 is still worth it's price right now, as in today...price will depreciate very soon, so decision is crucial...