You are at a dead end. All the new cards are PCI 2, and you have PCI 1 slot none of the NVIDIA will work properly.
The new PCI 2 ATI cards like the 4780 (or 4870 I get confused) is a PCI 2 but with ATI they seem to work fine, at as fast as in newer system but faster than the older cards. For that budget you would have to get a PC version and flash the firmware, read success stories but I don’t think that is for the timid.
Color cannot work with 2 cards, no problem with dual monitors on the same card.
For Color and Motion right now ATI cards are better. You may want to try to find an ATI X1900 that is used for about $100 but graphic cards are not really a thing I would buy used and the new X1900 is more than the 4870/4780?.
Don’t worry to much about the port there are adapters.
All that to be said you (and I’m in the same boat) are at a dead end with those systems, they still work fine but there is not much up grades for them, with CS5 and eventually when the next FCS comes out, they need the new video cards to really fly, software will work but just not at full speed. I would save the money and get really to buy a new system within the year.
My 2 cents