haha who needs a voltage mod when you get bust out the 3.3V! well it would definitely not be possible unless you smash your video card into the old slot... the new cards are keyed to fit only the motherboards they are meant for to prevent damage... so no easy 3.3V mod today... [insert smell of burnt pcb here]
I don't claim to be an expert on this, but I have done some recent research on the subject. The older 1X and 2X AGP slot run at 3.3v, the 4X AGP slots run 1.5v, and the new 8X AGP slots run at 0.8v. The cards are typically "keyed" (have notches) that prevent a 2X vid card from getting inserted into a 4X slot for example, but a 4X vid card can go into a 2X slot (see link). Sounds like "most" 4X cards are backwards compatible and will work in a 2X slot, but it will only run at the 2X speed obviously. I'd check with the vid card manufacturer first. For example, the Abit GF3 Ti4200 says it is 2X/4X compatible, so I'm assuming it would work in both 2X and 4X AGP slots. Is that clear as mud?
Err well now I'm confused I'll just use those pics on that page to find whether or not my vid cards will work in a 1.5V slot. If not then I guess I'll just have to get a new vid card when I get a new board
If you have a 4X 1.5v AGP slot on your mobo, then you'll need a 4X 1.5v vid card (that has two notches). The older 1X or 2X 3.3v cards will not work. Most newer cards will work in older machines, but not vice versa.