Hi Denim,
OK, that is an older Pentium 4 motherboard with the Intel 865GV Chipset, and will only support PCI devices (not PCI-e). The issue with this is going to be limits of the PCI bus - either watts or bandwidth, and I'm not sure, but the Dell motherboard may have some proprietary stuff there, so that might be the issue.
Unfortunately for crunching purposes, this is a small form factor board, and has the 3 PCI slots right next to each other just below the CPU. With the 3 PCI slots so close together, I'd be inclined to leave it with just one GPU in the top slot. You might get by with skipping the middle slot and putting a second GPU in the bottom, but then how close is that to the bottom of your case?
I think we're going to run into heat issues big time if we try to do much more...may be better off selling the other 2 GPUs to anyone who needs an older PCI slot GPU to fill an empty slot, or for older server type mobos that didn't have PCI-e slots, etc.
Hope that helps a little.