you have 305w PSU and that 8400gs can be powered by candlelight, you are all set
If you haven't bought the card yet... I suggest you get the HD 4350 instead, it's three tiers higher than the 8400GS, has very low power consumption, is passive just like the GeForce 8400GS. $34.99, right now at $14.99, deal ends today.
What kind of games will you be playing on your computer? You can go as high as the ATI HD4670 on the Dell Dimension E520 (305w PSU) and the performance will be huge from the 8400GS or HD 4350. It's worth the extra money.