I found that a 50mm fan fits the V3 3000 heatsink almost perfectly. The mounting holes in the fan almost perfectly line up with the mounting hole in the heatsink.
My V3 3000 has one of the bare aluminum heatsinks that is attached to the video card with two metal pins. The heatsink has 4 holes, but only two of them are used by the pins. So What I did was to line up the holes in the 50mm fan with the two open holes in the heatsink, and run twisty-ties through both the fan and heatsink to hold the fan securely.
If you wanted to, you could also remove the two metal pins holding the heatsink onto the card, and use long screws to secure both the fan and heatsink onto the card.