From what i know 2 x 4850s is slightly better than 1 4870 and is slightly worse than a 4870 x2.
Having two video cards in SLI/CF i believe up-scales the preformance but isnt a 100% increase in preformance. So having 2 4850s could Dramaticly increase your FPS in one game and not increase at all in another (it won't double your FPS). Results vary but you wont be getting 200% preformance from the two video cards, but more likley 150%. A second video card now or later on down the line is recommened by many people and by me.
You can crossfire any 4850 from any brand. However CFing a 4850 and 4870 isnt advised as the 4870 will only run as fast as the 4850 (basically having 2 4850s).
I run 2 Visiontek 4850's in a crossfire configuration (although crossfire is disabled the majority of the time so i can run 3 monitors). I'll try to cover all your questions.
1.) No adding the second card doesnt double the values of the card. Running two 512 meg cards in crossfire is still like having 512 megs of ram. Clock speeds arent increased but instead you have multiple GPU's (like the difference in a single core processor and dual core). The increase from running crossfire is nowhere near double. You are lucky to get a 50% gain at most and this is only if the particular game can utilize multiple GPU's, alot of games get no performance increase from crossfire.
2.) You can crossfire any of the saphire cards or even a different brand.
3.) Check ATI's site but you can crossfire alot of different cards but the performance gain is going to be limited by whichever card has the lowest memory / speed.
4.) Yes you can still use both outputs on the primary card when crossfiring. This means you can use the HDMI out and the DVI out.
If you havent bought your second 4850 yet you might want to consider a 4870 or even higher. I have seen the 4870's going for only slightly more then the 4850's after mail in rebate.