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