My 2 cents:
1) spend some time listening to sound samples on YouTube. Here is the obligatory E1M1 video. Here's another classic video, showing the evolution of sound hardware with Secret of Monkey Island. Here's a video that might make you fall in love with AdLib/FM music. Here's a website that has samples for some less famous cards.
2) Here's a nice article to give you a general understanding of how sound standards evolved in PC games. For instance, if your main interest is mid-90s FPS games like Doom/Duke Nukem 3D, you don't really need to obsess about AdLib and MT-32, because those sound standards became less important after 1993. So you just need a good MIDI device and a SoundBlaster clone with clean output for those games.
3) Use Mobygames database to check sound support for different games. Each game also has a "specs" page where all the sound standards are listed for each and every game.
4) Make a list of games that you really want to play. Literally everyone overestimates this part ("I'm going to play all DOS games from early 80s to late 90s" — no, you most likely won't). Also important is the operating system you're going to use. AWE32/64 really calls for Windows 9x, while GUS is mostly a DOS card. Some options are versatile, though.
5) pick a card that covers all your needs. Worst case scenario, pick two (e.g. AWE32 and GUS or SB16 and MT-32) — it's pretty easy to use two cards in the same system. Three cards in a single system is already challenging, and everything beyond that is a path to pure madness where you start rebuilding your entire computer around your sound hardware.
6) BUY THAT CARD. I mean, get the card you actually want. Try not to fall for compromises, don't get a cheaper card while you're waiting for "a good deal". You'll spend more time/money that way. For example, if you need a SoundBlaster 16 for a Waveblaster card — look specifically for a CT2230 and don't settle for a cheap Vibra.