After many hours of lurking dell forums I discovered I didn't need a new PSU to run my new card, like I'd thought. The 9800 gtx I got works fine with my stock PSU 12v amperage, I just hadn't hooked up the GPU additional power cables correctly, so not enough power was getting to the card simply from that.
I eventually found the right cable coming from the PSU -
geeze, it's a fortress of wires and bits n pieces inside my case, very confusing to a non hardware person - and hooked it up.
Rebooted - no more warning from nVidia about cutting the performance...
First thing I did was boot up SHIV to test. This seems to be the most strenuous game I have on my PC at the moment (COD4 as well I guess), so it's a good tester, lol.
Turned on every graphic setting available, and loaded up a mission.
My god
60FPS on the bridge and everywhere else (Think it auto selected vsynch when I set high settings, so it's halting max FPS at my monitor refresh rate). Wow, it's like a whole new game now. I might actually play it again, lol...
Just wanted to say thanks for the inspiration to upgrade, subsimmers.
To the OP again, my processor is not that great. But with
just this graphic card improvement, it has made a
dramatic difference. I really recommend an upgrade in that department, to whatever you can justify spending on it.
