you can mix the brand. also you can mix card with different amount of memory but i think you might want to avoid that. get the one that have the same amount of memory with your card because if not you'll be wasting the extra memory the other card have with it
If you don't match them, the faster card will slow down to match the slower one.... (kind of a waste....)
Or, if you don't match the RAM size, you'll only work with the amount of RAM on the smaller RAM size card... (again a waste....)
What I would point out about Crossfire (or SLI for that matter) is HEAT. The top card in the pair will run Hotter. Between 10C and 15C hotter than the bottom card. So, be sure you have some good ventilation in your case.
You current card dumps most of the heat inside the case. (I have a pair of Sapphire Vapor-X 5770's in crossfire. And while they are cool running single cards, they dump a lot of hot air inside the case, just like yours)
The other card you show vents most of it's heat Outside the case (which will help keep things cool). And the fans on each card don't line up, so, they won't both fight each other for air from the same point. This card would be a better choice for you to keep things cooler...
And finally, you'll need a good 650 watt power supply.....
CrossFire cards must only be from the same "family" in you case you Could pair:
For : "57xx family"
- 5770 with 5770
- or 5770 with a 5750 (odd, but it would work...)
Same for : "58xx family"
- 5870 with 5870
- or 5870 with a 5850
Only thing you Can't do, pair a 57xx card with a 58xx card ......
Your 1st link is to a GPU 850mhz, 1Gb RAM @ 4800Mhz
The 2nd link shows no specs, but, that fully enclosed plastic shroud is a reference card design. They run at the same speed listed above. (only those with fancy fans & coolers can run faster than that...)
I'd go with a fan at the front blowing cool air in and across the graphics cards....
Leave the one at the back. But the one on the side you might try running without it once you get the new power supply and 2nd video card installed.
Basically cool air in low at the front and hot air out high at the back.... (as hot air rises...) So that side fan might disturb this natural flow.
One note: with a top mount power supply with it's own fan mount on the bottom the Power Supply will suck a lot of hot air from the CPU and heat from the GPUs THRU the power supply. While this isn't a bad thing the power supply runs a bit hotter.
In a case I have with top mounted power supply, an Antec True Power 650watt :
- the power supply is dead silent with a single 5770....
- but, when I added a second 5770 the extra heat form the car caused the fan on the power supply to run Much faster and thus louder.
NOTE: I changed to a case with bottom mounting for the supply where the power supply pulled in cool air from outside the bottom of the case and runs silent again. So while you don' t need a 750watt supply for the power, it will likely run quieter in your top mounted case.....