However, if you just want to expand your desktop, and not going to be using other monitors for gaming, it might actually be cheaper to buy a cheap vid card rather than a whole comp.

Also, most laptops have at least a VGA out, if not also a S-Video out, which gives at least one option of having a second monitor.

After searching and finding info on MaxiVista, it seems it feeds the second comp a live stream of whatever is happening on the main comp. So I guess in the case of an extended desktop, a virtual second monitor is create on the master computer, and the slave simply mirrors that virtual monitor.

However, there’s a couple pitfalls to this software, in addition to your mention of the limit on the number of comps:
1) Windows only, though it can apparently work on Linux with WINE.
2) Doesn’t seem like you can actually use the second comp. It seems like any slave comps act as dumb terminals, simply mirroring what’s happening on the main comp.
3) Since it’s sending full images from one comp to the other, it’s going to use a LOT of network bandwidth. I can only imagine the amount of chaos on a network when there’s 3 slaves getting images from the master. Perhaps this is the reason for the limit of a total of 4 comps (1 master, 3 slaves).

I would say that if the other comps you’re using with this software are junk comps that you’d otherwise not use, then go for it. If you’re actually going to use the other comps, then Synergy is going to be more useful for you.