Just from my own experience, although others have had great success with the SP3, I haven't. I've loaded SP3 three times now, twice on purpose and once by accident when it was included with another update. Every time, my computer started doing awful things (including not booting, screen problems, sudden shut-downs, etc.) and each time I did a 'system recovery' and everything went back to normal. I think my machine just doesn't want the SP3 update for some reason. Love system recovery option!
Good luck with yours. I'd do the 'recovery' and never load SP3 again.