There's a bunch of few-years-old threads about the XG soft-synth on here where you can find all sorts of information about the various versions, but you have to wade through a lot of posts. I was looking at some of them earlier and picked up some tidbits. Here is a section from a modified Yamaha README file that I found elsewhere, from an install package, that covers the various soft-synths. These are obviously not the VSTi or WDM versions of the XG50 and are the "retail packaging", so they differ from what most seem to be using at this point.
=========== S-YXG100 PVL =============
Tone generator section
Tone generation method PCM / WAVE TABLE
Number of parts 16 parts
Polyphony 256 notes
Number of sounds : 676 instruments + 21 drum kits
(XG mode: 480 instruments + 9 drum kits + 2 SFX kits)
Tone generator modes XG / GM / TG300B
Dynamic filter section
Filters Dynamic filters for each note
Format of Sound Engine S/VA Synthesis System (VLR algorithm)
Maximum Polyphony 4 or 8 (Last in First Out priority), depending on your PC.
Number of sounds 137 VL-XG instruments (included in the total of 256 preset instruments)
6 Custom instruments
64 Internal instruments
=========== S-YXG100 (For Windows98) =============
Tone generator section
Tone generation method PCM / WAVE TABLE
Number of parts 16 parts
Polyphony 256 notes
Number of sounds : 676 instruments + 21 drum kits
(XG mode: 480 instruments + 9 drum kits + 2 SFX kits)
Tone generator modes XG / GM / TG300B
Dynamic filter section
Filters Dynamic filters for each note
Tone generator section
Virtual Acoustic Synthesis
Maximum Polyphony 1 note
Number of sounds 256 instruments
=========== S-YXG50 (For Windows98/2000) =============
Tone generator section
Tone generation method PCM / WAVE TABLE
Number of parts 16 parts
Polyphony 256 notes
Number of sounds 676 sounds + 21 drumkits / SFX kits
(XGmode: 480 sounds + 11 drum kits / SFX kits)
Tone generator modes XG / GM / TG300B
Dynamic filter section
Filters Dynamic filters for each note
Effect section
Reverb 11 types
Chorus 11 types
Variation 43 types (distortion, delay, etc.)
============ S-YG20 =======
Tone generator section
Tone generation method PCM / WAVE TABLE
Number of parts 16 parts
Polyphony 32 notes
Number of sounds 360 sounds + 9 drumkits + 2 SFX banks
Tone generator modes XG / GM
Effect section
Effect types Reverb