Unless encoding/transcoding is the bottleneck, odds are pretty low a GPU upgrade will improve the situation much. With 21 camera, you're most likely blocking on IO -- that is, the CPU copying data from USB to memory, then memory to disk.
On the input side, a small win might be to make sure all your cameras speak USB 3.0, USB 2 and earlier used a really cpu intensive polling model for data transfer. USB 3+ greatly reduces the amount of polling the CPU has to do. You might also take some time to profile the system while in operation to see where all your CPU time is going. (Unfortunately I haven't the foggiest on how to do this in Windows)