OK, this really is driving me nuts! I'll try to make this as quick as possible.
MSI P55-GD65 (1.7 BIOS)
4GB (2:2GB sticks) of Corsair CMX4GX3M2A1600C9
Pretty good CPU cooling
I can run this thing no problem at 3.4 GHz (20 x 170), memory at 1700. Nice low voltages - Core 1.182, VTT1.114, PCH 1.100. This setting runs pretty cool (28-31 at idle and roughly in the high 50's at full load - Prime95). XP see's 3.398 GB ram which I'm ok with.
If I go to 3.6 (which is a royal pain to get going at many BCLK x Multiplier settings) or 3.8 (19x200 - which is much easier for some reason), XP will only see 2.098 GB of RAM. I can even use the same voltages I use for 3.4 GHz at 3.8. I just can't figure out why the available RAM XP see's drops from 3.4 to 2.0 GB. I've tried different vCore settings at 3.8 GHz. The one below is just where it was at when I filled in the chart.
I'm sure you guys need more info. I included the settings below.