The difference in memory sizes wont matter much at your resolution, but could help if you upgrade your monitor. Then again the HD5770 would be the right call if you may upgrade your monitor. Keep in mind that card also is DX11 and uses significantly less power than the HD4850. That card for $130 after rebate really is a nice deal.
Another option would be the GTS 250. This is a particularly nice version with a big rebate if you like that kind of thing;http://www.newegg.com/Product/Product.aspx?Item=N82E16814127495
It is slightly faster than the HD4850 but still a tier below the HD5770