Neither card creates a power issue
8800GT = 58 watts (256 MB)
5750 = 36 watts
Source = http://www.antec.outervision.com/PSUEngine
Switching GFX cards however requires that you change drivers....this is "safe way":
1. Download latest ATI driver to HD; Uninstall Nvidia drivers, reboot
2. Clean out any stray registry items w/ a driver cleaner or registry tool like CCleaner, reboot
3. Remove nVidia card
4. Install ATI card w/ internet cable unplugged.
5. Install downloaded ATI driver from HD.