Can ati radeon fit with geforce 7100

Only 1 AGP device can be used at a time, that means your ATI will not work if you decide to use the GeForce. I'm assuming they are AGP.

Your better option is just to get a 4650 or 3850 AGP card and use that as your upgrade. You cannot combine an ATI card and an nVidia card, especially not on an AGP system, and the output would have nothing at all related to the video memory.

you can only do something like that on hybrid SLI / hybrid crossfire board. for nvidia you need nvidia IGP pair up with discrete Nvidia GPU to make it work and the same principle works on Ati side. but you can never mix between nvidia and Ati part.....