I don't understand how the 4870 is a candidate when he explicitly stated he wants a cheap low power card for 2D/desktop/internet apps. Nowhere does the word "games" or "D3D" appear in the OP. Simply because his budget is larger than it needs to be doesn't mean that he should blow it all.
I would suggest the radeon 4350. Cheap, passive and plenty of power for HD video. No reason to spend $130 on a 150-watt 4870 when a 35-watt Radeon 4350 will do the same job for only $30. In the above-stated usage scenarios, the difference between a 4350 and a 4890 or even a 5870 will be entirely indiscernible, except in regards to cost and energy consumption (two explicitly stated concerns).
If gaming on this machine is a distant possibility, I would address that bridge later on when it's time to cross it (like when RV840 is $100). For now, get your $30 card and move on.