1. They'll work, as they were tested out by Areca and placed on the HDD Compatability List (that list is your best friend when buying drives, as it can save endless headaches).
2. Maybe, as MacDrive has created problems for users before. But I can't say directly, as I've never tried to do something like that. I'd go for separate arrays (even if it means data duplication).
3. Go ahead and load EFI (safer).
4. I'd expect the card boots BIOS (none can boot both), so attach the Windows disk to it, and the OS X disk to the connector for the avialable optical bay.
5. See above.
ATTO and Areca both make excellent products (use nearly identical designs, as they base their products off of the same reference designs for the semiconductors used).
This means both know what they're doing in the Support Departments (both speak English BTW, but calling NY if you're in the US is easier).
ATTO's managment = custom interface software
Areca's managment = Access it via a Browser (BTW, use FireFox, as Safari doesn't play well with others).
Both cards use NVRAM solutions, and have features that you won't necessarily find from other manufacturers (i.e. copy of the Partition Tables in the ROM = recovery technique that can allow recovery of an array that would be gone with other cards). It's why they're usually considered the best 2x companies in the business in both performance and reliability (I'd take either company's products over Adaptec, LSI, or AMCC/3Ware).
Areca just came out with their 6.0Gb/s products (Aug. 9, 2010), which is the 1880 series. The difference is, you can actually get hold of an Areca right now (here), as it seems the RAID versions of ATTO's products are still hard to find (non RAID versions, which start with an H, are available).
If you want to gain some idea of what to expect out of it, look here.
