I know of this. But apparently theres graphic enhancers out there that you need to download and not install manually.
Graphic enhancers don't count as graphic cards.
I'm guessing from what you're discussing, you're thinking along the lines of something like Riva Tuner, which lets you increase the core speed, the shader speed (if applicable, seem my card won't let me adjust them independently), and the memory speed of the video card in the system.
This (usually) voids your warranty because it (usually) counts as modifying the card to perform outside of its manufacturer specified settings. It is exceptionally easy to set your card to specifications that will cause it to destroy itself, usually due to over-heat.
That being said, those do exist. I'm using one right now to halve the power of my video card which is helping to reduce my system's temperature (not by much, but by enough to be worth it). I turn it back to normal for gaming but otherwise leaving it at half power helps somewhat.
I also need to stress though - you aren't going to be able to turn a geforce 6 series card into something with the power of a 9 series card. The hardware itself limits what you can do.
Think of it like an engine in a car - you can tune it up all you want, but it does have a maximum output no matter how much tuning you do, and it WILL at some point need to be replaced if you want to crank the output up higher
But yeah, now that we're all on the same page (graphics enhancers/tweakers, NOT the card itself), we might be able to discuss it a bit better
Gotta admit I'm no expert on this either. I just know Overclocking can kill a cpu if you don't do it right, and thats no different for a gpu.