K6III (Plus?)--whats up with the "plus"? Is that a "mobile" chip?

[John Prophet]

I am looking for an upgrade path for an older (2nd machine) 486 AT case machine. SO I am considering K6III+. Ive heard the "plus" is much better than the nonplus. I dont see anything on the AMD site referring to "Plus" on any of the k6III's. I do see K6III(HX). Are they referring to the "HX" as the same thing as "plus". Havnt seen "HX" on any mobo listings. Is "plus" referring to a mobile type chip...Im not seeing K6III "plus" listed on certain mobo specs but have seen in certain forums that it will work anyway. Whats the deal on "plus"? Is it mainly a core voltage thing?
Am considering Epox EP-MVP3c...DFI K6BV3...FIC VA-503+

Thanks