Regarding receiver dimensions, they're exactly the same regardless as to whether the Hi-Power is a 9mm or a .40. So in terms of grips, they're identical (or dependant upon your grip of choice). The general concensus seems to be that Craig Spegel grips have the thinnest profile, and Hogue's rubber fingergroove grips the thickest. I personally prefer the Hogue grips, as they seem to do the best job in properly positioning my triggerfinger-but that's just me. There are a lot of good choices available; if you use the "search" function, you'll find plenty of discussion and pictures.
For holstering, keep in mind that the .40's SLIDE is a bit taller and wider-this is usually not a big deal with a leather or horsehide holster (both 9mm and .40 guns seem to work interchangably in the same holster), but in a kydex holster (which will be more specifically molded, and of less flexible material) you should specify exactly which one the holster is for.
Magazine exterior dimensions are exactly the same for both calibers.
Best, Jon