I currently have an intergrated graphics solution in my new computer (Geforce 6100) but i have an Geforce Fx 5200 in my old computer (both are 256mb). Can i add this to my current computer and if i can, is it worth it?

I don't think it'd be worth it. I doubt you'd see any measurable performance improvement; I'm tempted to think the GeForce 6100 Chipset has a faster graphics core than the GeForce FX 5200. It definitely has a better feature set in the DirectX 9c/Pixel Shader 3.0 support.

Neither of them are very suitable for gaming anyway, so you'd gain nothing and lose nothing.

I have integrated video too (GeForce6150) and i think that 6100 would be faster then 5200, but... External video usually have a better 2D quality and save 256 mb system RAM (integrated vide use your system RAM for frame buffer)

You raise a good point with the Video Memory consuming physical RAM. Users with integrated graphics are better of setting the shared RAM to the minimum. A GeForce 6100 could never make effective use of 256 MB of texture memory; 32~64 would probably be optimum.

It's not generally true that external graphics chips have better 2D. This was true maybe 3-4 hardware generations ago, when integrated graphics didn't even hardware accelerate vector surfaces or mouse pointers. A GeForce 6100 is feature-superior to a GeForce FX 5200 in nearly every way.

Most GeForce FX 5200 would be slower than an integrated GeForce 6100 (with the exception of the GeForce FX 5200 Ultra) in both core and memory bandwidth.