There is no danger of a too big PSU. The PC components only draw what they need. Any reserve makes for a poor man's power conditioner or UPS (only good for a split second of AC loss.)
Another analogy is a car. Run your car at 50, you get so many miles before a breakdown. Drive at 100, you get less miles.
I'm looking to upgrade the graphics card that came with my computer. I currently have a GeForce 7300 LE and am wondering if I have to stick with Nvidia or if I can switch to ATI. Do some motherboards only support some cards. I have a PCI express and two PCI expansion ports. The rest of my stats are:
It's an HP Pavilion a1650y series
Windows XP Media Center Edition 2005
Pentium 4 3.06GHz
2 GB DDR2-533MHz dual channel SDRAM
80GB 7200 rpm SATA Hard Drive
128MB NVIDIA GeForce 7300LE, w/ TurboCache
802.11 b/g Wireless LAN PCI Card
HP 19-inch LCD Flat Panel Monitor
Also I currently have a 300 watt PSU, the graphics card will need more power is there any danger in getting a PSU with too much wattage, say 650. Thanks alot for any help I really appreciate it.