1) If you fill all of the fan spots in both cases, you end up with more millimeters of fan in the HAF X than the DF-85 (1170 v 1000). The HAF X also has a front air duct that directs air over the graphics card, as well as a fan mount on the graphic card supporter, keeping one of the build's hottest components cool.
That being said, cases are personal preference. From reviews I've read the two perform quite similarly, if you like the looks of the DF-85 better, go for it.
2) Yeah the OCZ Agility 3 has some bad reviews, they also have quite a few reviews. When you sell as many SSDs as OCZ occasionally you make a lemon. The 120GB OCZ Vertex 3
is 4/5 instead of 3/5, but slightly more expensive and faster. General consensus is that OCZ is currently king of the castle when it comes to SSDs.
3) Not sure what you mean about different types of RAM. I'm suggesting 8GB of RAM, either the 8GB CAS8 kit in the first suggestion -or- 2 sets of the 4GB CAS7 RAM -or- 2 sets of the 4GB CAS6. At no point did I suggest mix-and-matching them, although you certainly can, they will just default to the slowest RAM that is currently plugged in.