Well, considering that you want to use two HD 5850s, then you'd have to get a new PSU if yours currently is an Acer OEM, and if you get a new, regular size ATX PSU, it would fit fine into any mid tower ATX for two HD 5850s in CrossFireX, AMD recommends that you use a 600W PSU or greater, with four 6-pin PCI-e power connections. One I recommend is the Corsair HX650W.
Any mid tower case would fit your needs, you just need to make sure it fits those two HD 5850s, and make sure that if say your motherboard is ATX, that you get an ATX case or bigger. A good case would be the HAF 922, for $89.99 on newegg IIRC - it has lots of room and more than enough space to fit two HD 5850s, and AFAIK it can even hold HD 5870s. Concerning graphic cards, Enzo is right - different brands will mix fine, but say if you buy an overclocked version of the HD 5850, and a regular version, the overclocked version will normally downclock itself to the speeds of the regular version. Also, ATi cards can only CFX with the same series number, i.e., the second number of the card has to be the same for both cards that you'd want to CFX, for example, HD 4830, HD 4850, HD 4870, HD 4890 and HD 4870X2 will CFX, but not say an HD 4770 and HD 4850, and that is the same with the ATi Radeon HD 5000 series cards. The HD 5770 will only CFX with the HD 5770 or HD 5750, and the HD 5850 will only CFX with the HD 5850 or HD 5870.
In terms of CPU size, I'm not sure what you mean, but as for airflow - it is quite important if you are overclocking for example and need to make sure that the CPU keeps cool. The HAF 922 has plenty of airflow, and it wouldn't be a problem.
As for the motherboard in your OP, which CPU do you have?