The specifications of the K6-2+ are a maximum voltage of 2.1V.
I would tend to stay at that, though if you wanted to try a higher one, go no further than 2.3v, but I wouldn't keep it that high.
You could add some devastatingly good cooling instead...that would be my option.
Better still, get a K6-3+ 550, they work rock sold up to 675MHZ with normal cooling, and a bit beyond that with really good cooling.

try www.computeradjusters.com for a K6-3+ 550. <IMG SRC="smilies/cool.gif" border="0">