It really depends on the board. If it supports the lower voltage requirements of the K6-2 (usually 2.2v), you should be able to slap in a K6-2/400, possibly a K6-2/450.
The trick with all K6-2's is that the 2.0x multiplier setting is recognized at 6.0x, so even if the board only supports up to 66MHz FSB, you should be able to run a 400MHz CPU (6 x 66MHz = 400). If the board supports 75MHz FSB, you should be able to run a 450MHz CPU (6 x 75MHz = 450). The CPU voltage is the important thing though...
If the board is based on an old Intel chipset...430FX, 430TX, 430VX, it would be best not to install more than 64MB of RAM. It will run with more, but the cacheable limit is 64MB. Exceeding the limit will hurt performance.
The 430HX chipset also has a cache limit of 64MB, but IF the board is equipped with the proper "tag RAM chip", the cache limit goes up to 512MB.
http://www.geek.com/procspec/chipsets/s7intel.htm