Problems with display changes HD4850

I recently purchased a Sapphire HD 4850 with 512MB RAM. I installed it and it is crisp and clear. Here is the problem I have. If I enter full screen games or try to change any of the display settings, it will basically black out one of my monitors, freeze my PC, and drop my network connection for about 30 seconds while it seems to refresh everything. I have tried a variety of versions of the drivers. (I clean off all the old drivers each time.) I have tried the PCI-e 2 slot. I have tried connecting only 1 monitor. I have tried loading the whole Catalyst or only the drivers. No matter what I continue to have the problem. I'm wondering if it is a PSU problem or something else. Here is my setup.

Asus P5K Motherboard
Intel core duo 2.66 gHz
4 GB DDR2 800 RAM
Samsung DVD +/-R burner
Sapphire Radeon HD4850 512MB DDR3 Dual DVI / TVO PCI-Express Graphics Card
2 SATA 7200 hard drives

I am running this with this power supply which is admittedly pretty old.
http://www.newegg.com/Product/Prod [...] 6817103922 - Antec TRUE CONTROL 550 550W ATX Power Supply

Is there anything else it could be or can I feel secure that a more recent crossfire ready power supply with a bit more juice will fix the problem? Any other suggestions you can provide? Thank you so much.