The EWS64XL is an good card, when you don't need CD-music while playing dos games and don't care about the limited functionality of the wavetable header. You will need a dual boot or use the DOS of win95/98, as this makes it much easier to set the mixer settings. But when its done, its a great card, because it allows you to use all the effects of the DSP in dos.
On the other hand I don't know if it differs so much from the Guillemot card you allready have.
Another option is the Turtle Beach Tropez Classic, it has an ICS wavetable synth onboard, a real OPL3 and claims to be SBPro compatible.
I like this card, especially in WC Privateer it sounds great. But it's hard to get in europe.
I recently ordered a Miro PCM12, which should be similar to the SW-20 you have. But it didn't arrive yet.
For pure GM/GS/XG sound the SW60XG might be the best option (best sound quality, no noise from the card).
Has someone the Orchid Soundwave32? Said to be the only soundcard that can truely emulate a MT-32 (I doubt that).
@ Elianda:
Who dares to ask him? > http://kay-bruns.de
(...ehm, better don't mention my silly "mad german technician" comments...)
There is still dos audio freeware available via his site (great!): http://kay-bruns.de/freeware/index.html
Look at the GUSPnp eeprom config tool! Looks very similar to the one used for the EWS64.