I've recently made myself a new rig and have had really good results. My old system was based off an I5 HP laptop that I was able to install XP on. I used an RME express card interface to hook up my Digiface. I generally ran a 16 channel mix with 2 monitor mixes and plenty of FX. The load would be 25-30% at 128x1 buffer settings. I would see anywhere from 5-60 slipped buffers during a night.
I've had a DAW that I hadn't used much recently, so I decided to repurpose it to SAC. This is a 4 year old Core 2 Duo running XP. I can run the same mixes on this machine at 32x1 buffer settings with no slipped buffers. The load is exactly the same as the laptop. I guess I'm just amazed that the desktop did so much better than the laptop given that its 2 years older. The new rig was used for the first time this past weekend on two shows. This was after extensive testing.