As for sound cards, it heavily depends on the variety of games you are going to play. As you could notice, many Vogons dwellers have multiple soundcards and external sound modules.
If you don't plan going deep into soundcards topic for now, I'd suggest to stick with a SB16-compatible Creative Labs card with wavetable capability. AWE32/SB32/AWE64/AWE64Gold would do it for you.
You also could search for an AWE upgrade for your SB16, but those upgrades are more rare and tend to cost more than whole AWE cards themselves.
Otherwise there are many (dozens of) soundcard and daughterboard options. Here are some of the popular ones (with wide support in games) with ebay prices:
Gravis Ultrasound Classic/MAX/ACE/Extreme [card, sound and wavetable music] - from $80
Yamaha SW60XG [card, wavetable music only] - about $100
Yamaha DB50XG/DB60XG, or a clone NEC XR385 [daughterboard, wavetable music only] - from $20 for a clone.
Roland SCC-1/SCC-1A [card, wavetable music only] - from $200
Roland LAPC-I [card, semi-wavetable music only, though some games squeeze some synthesized SFX out of it] - from $300
Roland SCB-55/SCD-15/SCM-15 [daughterboard, wavetable music only] - price unknown to me
While AWE32/SB32/AWE64, Gravis Ultrasound and NEC XR385 are relatively easy to find, other enumerated cards are pretty rare.
