connect 8 projectors together to create a large image
Do you expect the large image to be seamless, both in dimension and in tone? Each is a separate challenge. Color-matching 8 projectors can be daunting, depending on the technology. As a general rule, you would overlap the images from the projectors and feather their edges. This costs you something in size, with the benefit of disguising mismatches to some degree. Some projectors have this blending ability built in.

In any case, it would probably be better to use a pair of 4-head graphics cards on one computer, rather than 8 separate PCs. Keeping 8 PCs sync-locked is not easy.

There are commercial products like Watchout that will perform the slicing and syncing of one image into multiple playback PCs, if you have the budget for that.