DieselUV,
a normal K6-III should run in any board capable of running a K6-2/550. I wouldn't go for the /550 however. The /500 is much cheaper, and not really slower. K6-2 performance does not scale very well up there, since the slow L2 cache at 100 MHz brakes it.
K6-III or K6-2+ with an additional level of cache running at full speed will improve things much more.
Now what mainboard do you have there?
Regards, Peter