I've never heard of GeForce3 having dual output. Nor do they have a pci version as of yet, as far as I know. Even if they do, you can't do opengl across both. Something like the GeForce2 MX dual, ATI Radeon 8500, or ATI Radeon 7500, you can.
if I got a GeForce 3 now, and later when I wanted to set up dual monitors could I just get a decent PCI card and hook the second monitor up to that? even if it is an older card.
You can get a GeForce3 now and add a PCI card to have dual monitor set up. My computer have GeForce on my AGP slot and ATI All in wonder 128 pro in my PCI slot. The computer works great.
I've had a few problems using dual nVidia cards to do dual monitor setups. The problem arrises from two cards using one driver with two different settings (ala nVidia unified driver). Use the GF3 with a non-nvidia card and you shouldn't have trouble.