MKV is about as vague as QuickTime - it all depends on the contents of the file. If you're ripping Blu-Ray and expect a G4 to play 1920x1080 MKV files at full framerate, good luck. On the other hand, any of the machines you're mentioning should play standard definition just fine.
One suggestion is to transcode (using Handbrake or the like) to a QuickTime format. VLC is a great program, but it's not as optimized for either Altivec or using the video hardware as is QuickTime in OS X, so try that. You can even open many kinds of MKV files in QuickTime directly using Perian, for instance, and while you do often need to wait until the entire file has been scanned before it'll play nicely, depending on the kind of video it'll often play with less CPU.
One suggestion is to transcode (using Handbrake or the like) to a QuickTime format. VLC is a great program, but it's not as optimized for either Altivec or using the video hardware as is QuickTime in OS X, so try that. You can even open many kinds of MKV files in QuickTime directly using Perian, for instance, and while you do often need to wait until the entire file has been scanned before it'll play nicely, depending on the kind of video it'll often play with less CPU.