No, you cannot. They have to be in the same series. Your only options would be 5830, 5850, and 5870. Neither the 5830 or 5870 make sense. Just go with a second 5850 if you need more GPU power. Two 5850's will be a great setup though.
nope. even with 5xxx series you can only CF your card with 58xx families meaning you can get it to work with 57xx or 56xx series. btw it is best to CF two identical card, in your case another 5850. if you happen to get 5870 you still get performance similar to running two 5850.