The 9800 is a minor development of the 8800, to most intents and purposes, they are the same card.
The 9800 'EE' variant uses less power than normal, but to do so Nvidia have been forced to reduce its performance in order to save that power.
Get the 8800, it's an old but still potent card.
Yeah, a Corsiar 550W unit is more than enough. The only real issue is driver support. Because the 8800GTS is an older card NVIDIA doesn't really bother testing it with it's newer drivers it seems. For example, recently NVIDIA drivers caused some issues with G92 based cards in games like Bioshock, but the recent drivers have only fixed said issues in the 9800 series cards despite the G92 8800 series cards being pretty much the same. As a result you may have to use older drivers for some games.
Anyway that said the 8800GTS is much faster than the 9800GT EE and is certainly worth the extra $8