Graphics Card Problem...

Hello,
I have been using a 512 Mb, Nvidia 7600 GS graphics card on a Socket 775 mobo, w/ Intel celeron 3.06 processor, and 1 Gb of DDR 2700 ram, on Windows XP Pro, with excellent results for Second Life's intense graphics, for almost 2 years now, when suddenly 2 weeks ago, everything slowed to a crawl on my pc and then my screen went black. I rebooted, and it only got worse, so i came up in safe mode with networking, and nvidia told me my card wasn't receiveing enough power. I bought a new Black Widow 550 watt power supply, and when i started up...the screen remained black. I again entered in safe mode, and windows said i had recovered from a serious fatal error and i could not function hardly at all...so i assumed my graphics card was bad and removed it, and the pc ran fine on my onboard graphics until i purchased and installed a new 512 mb Nvidia 7800 GS (no 7600's). When i put in the card in the AGP slot, and boot the pc..the screen goes black and the pc stays on. I don't even see the bios information to enter safe mode. When i take it out, the pc does fine on the VGA drivers...but i can't enter our open sim grid this way! Could someone please offer me some kind of explanation to what mite have happened?? We did have a bad storm that cut off our power about 2 days prior to all this happening, but it ran fine after..and continues to run fine with the onboard video, but i need better performance.
Thx bunches for any suggestions!
Moon