Now that is a tough one but here goes the 4770 is 40nm while the 9800gt is a 55nm which I do own. Both cards are specked at 105 (board max) and with your psu both cards will run well unless you are seeking low power for a different reason that your psu. The 4770 is the best performing card and I would buy it.
The HD4770 was specially made for low power consumption and to run cooler. So definitely its better in term of low wattage and cooling. Performance-wise the HD4770 is marginally ahead of the 9800GT in some games. So if you want to buy, HD4770 is the one.
The HD4770 uses less power both at idle and under load and is a more powerful card.
If you can afford it however the HD5750 is significantly better and DX11 compatible while idling at less than half the power of the HD4770 and using the same amount under load.
The 4770 is closer in performance to the 9800GTX/GTX+/GTS250 (darn renaming) than the 9800GT. Really tho your PSU isn't too bad and you could go with a more powerful card so you may want to look at a Radeon 4850 (which is increasingly rare) or a 5750.