there is a 625w version of the MODU82+
But 525w would be sufficient.
9800 GTX requires a 450W PCI Express-compliant system power supply with a combined 12V current rating of 24A or more (Minimum system power requirement based on a PC configured with an Intel Core 2 Extreme QX6700 processor)
also
[expreview] 9800 GTX power consumption - Overclock.net - Overclocking.net