Overall the HD 4870 is slightly faster than the HD 5770. However, the HD 5770 consumes about 50% less watts than the HD 4870 and has DX11. Depending on how many watts your power supply provides, buying the HD 4870 may also mean you need to upgrade your power supply.
Online games are generally not overly demanding since they generally tend to capture the largest number of gamers possible. After Effects? Don't know, I would say similar results.
Based on the above, I would go with the HD 5770 which probably gives you 95% performance of the older HD 4870.
thanks you guys I'm getting the hd 5770 ....... and i got a power supply of 450 watts------with a AMD athlon ll x4 3.1 ghz overclocked with a msi 785GTM-E45 motherboard so i dont have to buy a power supply if buy the HD 5770?