I'm not sure I would want to try running at 9800GTX, but I might consider some of the more power efficient cards.
The best card I would try would be the sort of new ATI 4770-- it is a pretty low power user. Safer yet would be a 9800GT, a GT 240 or a 9600GT. (9600 and 240 are similar in power-9800 is better) All would be better than a 4670, but would cost more.
The new GT 240s have a 300w power requirement, and the older 9800 and 9600s only have a 400w requirement, so I am certain an Antec part could handle them. Those requirements are made for cheap PSUs. http://www.tomshardware.com/reviews/geforce-gt-240,2475-19.html
Anyhow that was mostly just for fun--if this is a family PC, it probably doesnt need anything more powerful than the 4670.