HD 4850

no real difference, except help/support, cooler, and possibly clocks. other than that, go with the one that looks best or is the cheaper of the two

the cards are going to perform pretty similar. one of the differences is the cooling, and whether or not there is extra ram (1GB vs 512 MB) and if it has been preoverclocked. If it is pre-overclocked, you may try finding one that is not and OC it yourself. As for the ram, it depends on what monitor you are using, and im sure the other people here can offer recommended cards/screen resolutions.

thanks. for me this is quite useful information. I am using ASUS MW221u at 1680x1050, 60hz.

then i would say that 512MB editions of the 4850 will be fine for your resolutions.