I was a system builder in that era -- '90s. Probably the only MS-DOS games you'll have trouble with are '80s ones in CGA or EGA which were dependent on system clock speeds. Nibbler is an example of this type of game, which used the time that it took the screen to redraw to slow down the game.
The fix for that 286-era game was the 'Turbo' function of the 386 processor. What turbo actually did was halve the clock speed of a 386 when you disabled it. So, Turbo speed was normal, and no turbo was half. Without that speed cut Nibbler was close to impossible to play.
To cut down on support calls many builders would put a hard jumper on the motherboard forcing the system to be in normal mode. Accidentally hitting the button with no knowledge of what it was for was a more problematical support issue. Your Duron motherboard will not support 'Turbo' since they had long given up supporting odd edge cases of poorly coded applications which that feature addressed. For any game you are likely to want to play none of this will even be an issue.
The larger key is not getting XP on a system which loses direct access to hardware. XP was the first consumer line which has a Hardware Abstraction Layer (HAL). That is what broke most of the games of that era which you want to play, Shogo, Myst, Shadow Warrior, Early Doom, and Quake.
98 SE (Second Edition) is probably your best choice if you can find install media. It has the best rudimentary USB support. Some people online have reported problems with partitions larger than 40GB, I have not seen any problems on a good Motherboard. However if you have lots of small files, smaller partitions will help with slack space losses.
Due to BIOS (hardware) limits of the day ATA drives of the day over 137GB may not work.
My own retro system is a First Motherboard AZ11E/Duron 1000/ 256M RAM and a Diamond Viper II Z200(32m) video card. You might want a better video card since I don't ever want to play Myst again. I love Civilization 1.5 which is MS-DOS based. My OS partition is 20G and I have 2 full 50GB partitions for games and other junk.
These old systems are especially power sensitive. If your board supports the Duron it likely also supports the Athlon. As every one else has mentioned, yes it wants a big fan.
You also do not want to install Windows ME, or Millennium Edition. ME doesn't have the Hardware Abstraction Layer, but it was so buggy that you want to Run From It, not run it. ME follows the every other version rule for Microsoft OS's. They get it right every other generation. (Google image search for "Windows CEMENT")
Lastly the fact that your board had no fan could be because it was from a chassis that had a large 80mm or 120mm fan hooked to a plastic duct on the back of the chassis. All of the large manufacturers toyed with this build technique to save on custom components, but probably went back to putting the fan on the heat sink after the hassle of customers losing the processor after they lost the plastic shroud.