HD 4870, at 1600x1200, in Frontlines, gets 56 fps. HD 4850 gets 39 fps. You can play all right at 39 fps, but 56 fps is a real treat. Ideally, if you have an LCD with 22" size and 60 Hz refreshing rate, you want a card that gets as close to 60 fps as possible at 1680x1050. You can use benchmarks for 1600x1200 to approximate 1680x1050.
In Crysis the difference is much smaller (44 fps, 48 fps).
It really depends on the games you play. I suggest you find more benchmarks (Google "HD 4870 review") and then decide.