Hello, I recently bought a Radeon HD 5850 and it won't fit into my acer OEM case. I also want to buy a new crossfire-capable motherboard. Will any mid-tower case fit my mobo needs, or do I have to match specific models?
Take a measuring tape to the store with you to make sure the 5850 will fit. I can tell you that the Antec 300 will fit it... just barely. I'm not sure about other cases but if you are ordering online you will have to find reviews or something to make sure it fits.
As for your motherboard needs and the case, there is something called ATX. It is the standard form factor and all motherboards are ATX. A branch of that is micro ATX, which is shorter on the bottom. ATX cases fit both, and micro ATX cases will only fit micro ATX.
Anything in the same generation can technically crossfire (ie 3870 w 3650 or 5850 w 5770) but it is really not recommended and nobody does it. I believe the more powerful card will operate at the same speed as the slower card.
If you are referring to different brands of the same model, then yes it will be fine. But note some manufacturers have different clockspeeds so see the note above about that.
If you need any more help or clarification, ask away.
So the Antec 300 can fit dual 5850's? I'm looking for a case under $100 so it pretty much has to be a Mid-tower ATX case. Would power supply and cpu sizes be a problem? And how much should I be considering cooling/air flow? What would you recommend for my budget and build?
The antec 300 allows upto 11ish inch cards and the HD5850 is something like 9.5" i believe. So you should have bags of room in that. Although I'd suggest something like an Antec 900 two. The 902 allows upto 16" cards (with Drive bay adjacent removed, 13.something inces with it installed) which means you'll have bags of room in the antec 900 two. Also has good cooling, but can be loud with fans on full, but that doesn't bother me coz i'm always playing music.
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?