Depends on why it is freezing. If you are running a lot of PCI cards, and your board is too old to support PCIe or even AGP... there isn't much you can do. You only have so much bandwidth to deal with, and running a video card from plain old PCI... it will try to hog all the resources, and your other PCI cards will be hungry... hence it will freeze or stall because of IO and bandwidth requests.
What motherboard are you running? What is your CPU, how much RAM do you have? Something else might be causing the freezing.
Just to state and example. I've got an old Asus P5P800-MX with a Celeron D 336 2.8GHz, 1GB DDR400 RAM, and the onboard video sucks. It doesn't have AGP or PCIe... so I went with a PCI FX5500... my system would get stuck and do random things unless I install the video card in the second PCI slot. If I installed it in the top or bottom, it wouldn't fully boot, or would freeze later. (Note: Using Windows 7 32 bit)
This was a problem with a single PCI video card that didn't like being in the top or bottom slots, only the second one. But I have had problems in the past with Gateway P3 towers that sometimes you had to move around the sound card or video card if you install something newer. Before buying anything else, just shuffle things around a bit... it's free. But if one of the cards is eating up all the bandwidth... it'll freeze and not much you can do about it. Time to change the board or get a new tower. Video cards on PCI suck anyways. The PCI bus is a huge bottleneck.