IOpener knowledge kicking in...
Voltage
K6-2's are mostly 2.2V
K6-2+, K6-3+ are 2.0V
K6-2P was available in a couple voltages I think, around 2.0V
Because of the voltages, and correspondingly heat, I'd really try to get one of the +'s or a K6-2P if at all possible. If you've already got that 550 though, you might as well try it.
The +'s are actually pretty cool, and you've actually got pretty good odds on running one under 2.0V, especially if it's underclocked.
Multiplier
Fraction looks like it's multiplier
CXT cored K6's (which started around 300MHZ) will remap 2x to 6x. That means if you set it to 66Mhz x 2, you'll be running the CPU at 400MHz(66*6).
As KF mentioned, there's software for dynamically changing the multiplier of the +'s;
here are some k6+ utilities.
Also, see if you can figure out what chipset is being used. You might be able to use SoftFSB to change the FSB in software too.
Conclusion
Get a K6-2+ or K6-3+. They run cooler. They're faster because of the additional cache. They have nifty software for them.