I'd say 4850, unless you're really concerned with DirectX11 support. The 4850 is a much better gaming card than the 5nnn series cards available at that price. Check out this THG Best Graphics cards for the money article for a few more details.
The 5670 is much more an HTPC card than a gaming card, though it might perform adequately at 1440x900.
# PCI Express® based PC is required with one X16 lane graphics slot available on the motherboard
# 450 Watt or greater power supply with 75 Watt 6-pin PCI Express® power connector recommended (550 Watt and two 6-pin connectors for ATI CrossFireX™ technology in dual mode)
That power supply is pretty limiting. You don't really have a whole lot of options with a 300 W PSU. None of the current (5nnn series) ATI cards suggest less than a 400 W PSU. And they're generally less power-hungry than the previous generation. Perhaps there's an nVidia card, or something from an earlier generation available.