To use a video card in a Mac, the card must be Mac-specific (i.e., it has a Mac ROM on board) so you can't use just any off-the-shelf card.
Apple isn't selling replacement video cards for the dual-core machines (including the Quad) at this time, and ATi hasn't released a PCI Express video card for Macs either.
If money is really no object, you can order a "replacement" nVidia 7800GT from WeLoveMacs.com
here. Because it's a replacement part, and not really meant to be sold at retail, the price is more than double what you would've paid had you ordered one as a BTO option on the Quad.
Some people have also had success with having the strangedogs.com people flash the ROM of a standard PC 7800GT.
The Quadro might be a little bit faster than the 7800GT, but I don't know of any place you can find one...