Latitude D800 slows to a crawl when GPU fan kicks in

I recently had to replace the video card in my Dell Latitude D800 after a bad vbios flash. I couldn't find a Geforce 4200 (the card it originally came with when I bought it in Summer 2003) so I grabbed a Geforce 5650, which had twice the memory and a faster gpu, so I figured it would at least be a nice upgrade from my old one. It installed fine, and I got the drivers from Dell, and I'm not experiencing any video problems (aside from flash video on sites like Youtube and Gamespot taxing the system much more heavily than they should) - but when the GPU fan kicks in it's incredibly loud, and the whole system slows down significantly. Also, it seems like even mundane tasks like having a few tabs open in my web browser will cause fans to kick in, which never used to happen.

Any idea why this could be happening? I don't know if it's specifically the new video card, if it's the new BIOS (I had used A01 or A00 or some really old one until a few months ago) or Windows Search 4 which seems to have caused a memory leak in IE, or my PC just aging, or a combination of factors.

I'm using the latest BIOS version, A13 I think.