The service pack issue only matters if you're using a non-legit code. If it's legit (and you do have the sticker) then you're ok.
Really? I don't recall being able to install an OEM version of WinXP SP1 with a SP2 serial before, but I could be wrong.. anyway, there are definitely OEM and Non-OEM versions of Pro.
I agree though, if you're just MAME'ing the machine there's really no need for Pro.. I mean really, the differences aren't all that significant unless you plan on using it as a server as well.
Own Magic, Star Gazer, Batman Forever, STTNG, Mystery Castle (project)
Gone Fairy, Secret Service, Meteor x3, Title Fight, Eight Ball Deluxe, Bone Busters Inc., Seawitch, Starship Troopers, Strange Science, Arena, Hook, Pin*Bot x2, Time Warp, Motordome, Robocop, Black hole, Jurassic Park , Wipe-Out, Pinball Pro: Challenger I, Swords of Fury, Stargate, Party Zone
I have a win XP home serial number (doesn't indicate SP2) and I installed a CD containing SP2 built in and it worked, at least for me it did! Serial number was accepted ok, the big problem I had before (that was my 3rd try I think) was that I was trying to install a non-OEM Win XP home and my serial number was OEM. That's how I found out that little tidbit of info.