And what are you using for a PSU? I would also be concerned about the GPU fan and other onboard cooling but first things first.
Recently I tried to upgrade my graphics card, from an ATI Radeon HD 4350 512mb card to a 1GB Nvidia GT 220 card, which I had lying around in the house.
I was actually getting decent fps on the main game I play, team fortress 2, with the ATI card. But I'd recently purchased a 21.5 inch BenQ monitor which needed a better graphics card to run games on a high res. (I had previously been using what It think is a 17" old Dell monitor.) So I decided to hook up the nvidia card. However, I noticed straight away it wasn't right. After installing all the drivers, and uninstalling the ATI ones, I realised that the GPU fan wasn't actually functioning. I had plugged the fan in and it should have worked, however I booted up my PC without the side panel on and it wasn't. As soon as I boot up a game, the card rises to above 80 degrees, I get significant FPS lag, and even some quite extreme graphical glitches. So, I decided to go back to the ATI card. I sorted out all the drivers, and I assumed it would just go back to being how it was before I tried to upgrade. I put away the BenQ monitor and decided I would just keep it until I can afford a better PC. (Which is a while away, as I'm 16 and I can't really get a job.)
I quickly noticed that something wasn't quite right, as I booted up TF2 (with an FPS config too) and I was getting drops to 20 fps. This makes the game entirely unplayable. I also tried to play the Shootmania Alpha, and I realised I was getting between 30 and 60 fps, on the lowest settings. My PC isn't particularly good, but before I tried to change anything It was running tf2 and shootmania with no fps lag whatsoever. Here are my specs:
MD Phenom X4 9650 45 °C
4.00 GB Dual-Channel DDR2 @ 400MHz (5-5-5-18)
ATI Radeon HD 4300/4500 Series (ATI AIB)
Thanks for any help, I really miss tf2