Those Shuttle power supplies are generally good quality stuff and the wattage rating is probably somewhat conservative (as compared to some ultra cheap PSs that are marketed as 400+W). I used to run a 9800Pro in a Shuttle box with a 200W PS and didn't have any problems. I'd also guess that you'd have to assess what other components you're running in the box. If you've got 2 RAID-ed 10000RPM drives, with DVD burner, 4GB of RAM and something in a PCI/PCIeX1 slot drawing power already, you're more likely to have power issues if you add a 7600GT.