Sound card
The puny little pc speaker is not exactly known for its great sound quality, so a sound card is a must!
To ensure compatibility, it really should be an ISA card. Sure, there are PCI cards that works under DOS, but old games usually adsresses the sound card directly. PCI sockets were introduced in 1992, and most games older than that cannot address PCI sound cards, as they do not know about the address ranges used by PCI.
In an attempt to remedy this situation, the SB-LINK entered the scene briefly. This is a small six pin header, that is found on some motherboards (the iWill BD-100 Plus, among others) and sound cards (which could be the ESS ES1938S Solo-1). The header on the motherboard is essentially an ISA-slot, meaning the PCI sound card can be addessed through this header with ISA-addresses. While it does help some, there are still some games that does not work with it.
So, ISA. But which one? Well, you can never go wrong with a Sound Blaster, right? A decent Sound Blaster 16 should set you back about 50 US$ on eBay, but you can of course go for a higher spec card like an AWE 64.
It you ever owned a AWE 32, you probably have fond memories of it. Rightly so - it is a great card, but it does have a tendency to be quite noisy, picking up a lot of static from the other card in the case. The AWE 64 reduced this a lot.
Again, look out for golddiggers! I have seen some listings on eBay where people are asking over 200 US$ for a AWE 32. You can get them for about 60 US$.
As I'm writing this, there's a greek trying to sell his AWE32 on eBay for 775 US$. I know Greece have financial problems, but that is just ridiculous...