MM4/5 are an amazing amount of detail, depth, and gameplay for the price. MM6 is pretty huge -- although it is vulnerable to abuse if you know the game too well and want to really burn through it. On the other hand, they're all fun. I still remember playing MM1 twenty years ago, making maps on graph paper. :) Classic games, tons of fun, substantial replay value. The weakest point of MM6 when it came out was lack of support for hardware accelerated video (because, back then, the pallette tricks they were using for different monsters weren't widely-supported enough, and many cards didn't have the memory for the engine). Given that the game ran silky-smooth on a 200MHz i686, though, there's no worry about that now. MM6 lets you choose between a real-time system and a sort of semi-turn-based combat system; the latter has some unusual quirks, but it's still playable, and a huge amount of fun to play. MM7 and MM8 used the same engine, slightly updated, so maybe we'll get lucky and see them some time.
Be warned: If you have a widescreen display, you will almost certainly want to run in windowed mode -- which requires a 16-bit desktop rather than a 32-bit desktop. HOMM3 runs in 800x600, period. Something that may not be obvious from the description: This includes the map editor and campaign editor, which were one of the best parts of HOMM3. You can make your own custom maps, and the system is extremely flexible and adaptible. The provided maps and scenarios are pretty fun to begin with, too. While HOMM4 had some lovely upgrades (better flexibility about screen sizes, for instance), HOMM3 always felt like it had the best balance, to me. Good balance, every turn is interesting, always something to do...
