In that case, it might be possible to track down the 512 kB cache that shipped in the 6500/275 and 6500/300, but those would be pretty rare. I don't think there is a 1 MB cache available for those machines. I don't know for sure about clones, but a possible source could be something like a PowerBase or C500/C600 (as long as you don't buy the CacheDoubler, of course). Keep in mind that the cache SIMM for Alchemy/Gazelle (6400/6500) boards is not the same as the cache for TNT/Tsunami/PowerSurge (PCI Power Macs like the 7300 and 8600).
The G3 doesn't require any extra fans. G3s are relatively cool to begin with. But if you are putting it in cramped quarters (more cramped than the 6400 case), or you are blocking the air vents, you'll run into problems. The G3 is marginal on temperature without any extra help like that - for instance, I overclocked a G3 400 MHz in a 6500 to 500 MHz and suffered crashes consistently after an hour or two of use when the machine got too hot inside. Of course, the 6500 already has a fan on the 603e CPU, so they are already pretty hot inside.
Physically, they are a little bit larger than the L2 cache SIMM, but not by much. They are about as tall as the AV card. They are a little fatter than the cache because of the heat sink. And IIRC, they are a little bit longer. But ultimately, they are designed to be unobtrusive so that they fit inside even a 6360. I doubt they'll get in the way of anything you're doing that wouldn't already make them too big for a stock machine which have pretty cramped quarters to begin with.
If it's not a secret, what are you trying to do with the 6400?
As for 6100s, they top out at 1 MB of L2 cache. It's pretty awesome in those machines. It is expensive, though, and probably pretty hard to find in this day and age.
Peace,
Drew