I have Windows 7 64 bit, 4 gigs of corsair ddr3 1600mhz ram, a phenom ii 555 be that i unlocked to x4, a xfx 5770 that i plan to sell once i buy one of these cards, msi 785gm-e51 and im planning on getting a new psu once i get one of these cards, i have a 460w thermaltake psu right now.
Honestly, with the refernce cards, it's all about cosmetics. They are all built to the same specs. IE: Which one is prettier to you? There is someone attached to this thread who won't buy anything until the right color scheme is available...not saying any names but...<cough>...I'm sure he'll see this thread.
The main difference is warranty stuff. XfX has a double lifetime warranty where you can put the warranty in your name and pass it to someone else if you sell it. The saphire, I think, is a 2 year warranty.
IMPO, historically, Saphire has had a better product over the years. But when it comes to referenced 5870's, you're good no matter what you do. They're one of the best video cards for the money in quite some time. I love the freaking things. I have a rig that was a quad crossfired 5870 setup in it. Yes it's over kill, but it's just so sexy! About half the people on this thread have purchased 5870's from me and I haven't heard one single complaint yet.
The non-referenced card is great if you want to run water cooling to your GPU's. It has an open case and easy to take apart. The reference cards aren't the easiest/safest to take apart. I hear that the non-referenced are easier to overclock too.