The 4670 should run on a 300 watt power supply. The ATI statement of 400 watts is "recommended" only, not reqiured. Be careful with the 9800GT. One version requires a six pin connector and might be too much for the 300watt power supply.
However there is a "green" or lower power version that does not require a six pin power connector and should work on your 300 watt power supply. Actually I am running one of these on a 300w power supply with no problem so far (about 6months.) You might be able to find one of these on sale or close out for around the price you are looking for. Probably the best card would be the ATI 5670, but that card is new and still overpriced. A 4670 should be in you price range though, and is maybe 30% weaker than the newer 5670.