Well, myisamchk I believe is only used when repairing/optimizing tables, so the larger that value, the faster that process probably goes, so I don't think that is of much a concern. I haven't tested any different myisamchk values to be able to say for sure.
As for key_buffer, that may be a little high, but it's hard to say depending on where the rest of your memory is being used throughout the system. I personally use 64MB and 128MB for some of my 1GB ram systems, and find that works fine. I have not experimented with much higher values.
You may want to run a few comparisons, starting with 128M, checking server load, memory usage, and the mysql footprint, run a few intense queries and average the time. Then try 256M... do the same thing.. and again your current value, just to get a better idea as to what your system likes best.
