I had a set of Sapphire 5850 Xtremes crossfired and they overclocked awesome! From 725mhz to 891 fully stable and ran under load around 65C. Next time I'm in the market for a new GPU it will definitely be a Sapphire card.
I've never used Crossfire/SLI before so I don't know for sure, but I would assume that using matching cards is standard/best practice.
You don't need a sound card unless you really want one. Your motherboard has onboard audio. If you're just playing games or casually listening to music then you should be perfectly fine with the onboard audio.
Your motherboard will come with an audio port for your speakers.. if you're using any kind of basic setup (I have just 2 Bose speakers, one left one right that share the same port) you won't have any problems. My motherboard came with lots of other sockets for surround sound and stuff too, however, I don't use them. Bottom line.. unless you have a surround sound system or something fancy, as long as your using a fairly basic sound system onboard sound should be more than adequate for your speaker/headphone needs. You can look at the pictures of your motherboard and pretty much see the ports that are on it.