Well, technically you aren't talking about grid computing. You're actually just referring to cluster computing (grid computing implies geographically distributed systems as opposed to just several systems sitting next to each other).
Now, beyond definitions, you may get some faster video processing using a cluster. Or you may not. When dealing with clusters, the key ratio you look at it computation to communication. If there's too much communication compared to the computation, then you'll actually slow down by using multiple computers.
In actuality, it's rather hard to get a speed up by using clusters. The communication required provides a huge hit on your performance maximization. This is the main reason that we haven't seen many commercial level clusters. It's usually restricted to things that are termed "hard problems" (N-body gravitation, cracking encryption, engineering simulations, etc.).
The vast majority of clustering setups these days no longer do implicit clustering. There are some software packages that do things in that manner but explicit clustering is more preferred now. What this means is that software has to be aware of clustering to take advantage of it instead of it happening automatically. The reason for this is the communication problem described above: the software needs to know if things are going to other machines so that the communciation doesn't swamp things.
That said, I would highly doubt you'd ever get video games to work in a cluster situation. For one, most are single threaded and don't have their work partitioned such that it could even take advantages of multiple processors, let alone multiple computers. There's simply too much communication required compared to the amount of processing.
I'm doing my thesis in this area so I have a bit of knowledge.
Matt