you need to worry more about length than height of tower,,and room for double slot card,, just ensure no SATA connections or hard drives are mounted in the area. you will need 11 inches + a little extra for air movement from your PC slot to the front of case unobstructed.
Also, make sure you have enough width on your case so that the 2 power connections can be fitted to the board as the are on the side of the card and not at the end of the board. (3/4 of an inch should be enough)
Manufacturer decision is pretty much user preference as most will build close to AMD specs, just pick someone with a good history and keep an eye on specs and any aftermarket cooling solutions,, some will work better than others, but get something with 2 or 3 larger fans that will spin at a slower speed.
to be honest 1GB card is 'probably' good enough,, but you may want to go with a 2gb model just to ensure no memory/texture issues
I also highly recommend a card using 2 or more fans since you are using a multigpu setup, you'll need as much air as you can get and multiple fans are typically quieter as they don't need to spin as fast, I personally like GIgabyte,,it has 3 fans and comes with an almost 10% overclock but that card is 11.5" long and they only have a 1GB model,,
I would still say you'd be OK but there is another card if your budget allows