Certainly do not use SLI/Crossfire. Games do not fully utilize it, those cards don't have proper video acceleration, it costs more than a single card with more performance and you need an expensive motherboard. $120 new card > $130 2x very old cards
Ubuntu. Linux for human beings.
If you are interested in trying Ubuntu or Linux in general, PM me and I will answer your questions and help you install it if you wish.