The reason the CPU is probably maxing out is because that video card is not off boarding any of the cpu load..so the cpu is doing all the work. Normally a machine kind of balances the load between the two. When your video card is that old it cannot be used on OBS for encoding.

When you have a newer video card, and using OBS the video card does alot of the work.

If you can just buy a mid range video card and see if it works on the old PC it may make all the difference. If its incompatible just return it and then build a newer PC.. or save it for the new cpu and motherboard.