i'm building a new system with a amd 955 at its core but i already have 1 4850 with 512mb on it. should i get another to crossfire and use dx10 or get the 5770 for dx11 but lose the 256bit from the older card? i'm running a 24" monitor and want to take advantage of my 1080 res.
DX11 should not be a deciding factor, because by the time DX11 games are mainstream you'll probably be looking to upgrade that 955.
You won't get more than $80 for your 4850 if you choose to sell it, so either way you'll be spending $80-100. The better choice would be to get a second 4850, as a single 5770 may struggle at that res.