In some ways XP is starting to show its age, and I think mainstream XP usage will continue to fall, reason being many mainstream users also use their PCs for entertainment etc. which also include gaming, and gaming usually evolve at much rapid pace, hence, in the longer term, XP will become a bottleneck as it doesn't support DX10 or above, whereas, almost all the graphic hardware available does that.
Now to your question about XP x86 SP3, I've couple of machines at work running this, and there are no issues with regard to stability and / or speed.