What do you want to play? And what features are important to you?
A P75 is great for mid to late DOS stuff, but is too fast (and an OEM system like an IBM isn't great for clocking back) for the really old stuff. It's also a bit slow for the very last DOS games. Sweet spot for old DOS is about 486-33 (de-turbo for XT speeds).
For XP that AM2+ thing should fine if there's driver support, although tbh I run pretty much any XP game on my daily driver Windows 10 system with Core i7.
I also miss Windows 9x in that list.
So personally I'd suggest:
- 486-33 (DX, SX, doesn't really matter) for old DOS, with 100% SB (maybe Pro) and HardMPU if you want MIDI. No TSRs as games from that era need a LOT of conventional RAM.
- fast P3 or maybe early Athlon/P4 for Win98SE and last-gen DOS games (Quake...). Nice OpenGL/D3D GPU with decent VESA DOS support (Gf4Ti, Gf FX5xxx), add a Voodoo 1 for DOS GLide, or V2 for Win9x GLide. An A3D and/or EAX sound card would be nice, but most of those don't do DOS (or interesting MIDI), so possibly use two - say an SBLive/Vortex2 and a late ISA card for DOS if possible, otherwise a nice compatible one, Yamaha YMF74x or ESS Solo-1 or similar. In Windows you can have both cards active and switch between them, in DOS simply don't initialize/load drivers for the Live/Vortex.
By all means go for an XP system too for nostalgic reasons, but It's not likely to add much n terms of compatibility between a current system and that fast P3.
Of course, if you want more systems, you can go for more fine-grained steps, but these two (plus your current system) should give you >>95% coverage.