Recommendation:

- for old MODs (4-channels) should standard Soundblaster (Pro) card be enough.
- for my more channel XMs minimum is SB16 and about 50% of energy of your CPU. But I can recommend any Ultrasound card (or some ..hmm.. SB AWE model + good player with AWE native support). Or TerraTec EWS64 with Mod4Win should be the best solution in the future.
- for listening MIDI files standard Blaster isn't suitable. Yes, of course, I know software emulators of wavetable synthesis working under DOS or Windows, but software mixing produces scratch noices, cuts notes and not all the emulators can interprete all the notes and effects. What I can recommend are following WAVETABLE cards: Roland Sound Canvas (MT-32 has some incompatibilities in instrument maping), Turtle Beach (each wavetable model but one of the best is MULTISOUND), Gravis Ultrasound models in this order: PnP, Max, Extreme, Classic and ACE), EWS64 (hopefuly the best card in these days but I've no other information, about this one, besides information from manufacturer's homepage), some YAMAHAs and ..hmm.. Soundblaster models (AWE32 - p.s. AWE64 = cheap marketing trick - and SB32).
Different players (from manufacturers) play MIDI files differently. Windows Media player always doesn't play them correctly too. So that if the song sounds strange => try another player or sequencer (e.g. Cakewak Pro).
The same could be said about MODS, but there is always tracker where the MOD was written in. So that if you've troubles try play the file in it.