1)
A RAID controller might add performance, but this depends on the RAID controller and the RAID levels. A cheap controller might do worse. Vice versa I expect that a good controller with BBU and RAID6 level will do better.
2)
If you use striping (RAID 0) then yes, the OS should see a single 1TB drive. Depending on drivers and OS you might also see the separate drives.
3)
Before buying a RAID card, sit down and think of what you want:
- Do you intend to stay with 2 drives? Or will you add more at a later date?
- Do you ever intend to buy a SSD (in which case SATA III or SAS 6GB/sec might be nice)
- Which RAID levels do you want? Striping? Mirroring? RAID5 so you can survive a single disk failure while loosing only a single disks worth of space? (Check out this page for a nice clear description of RAID levels).
- Do you intend to install a second OS later on? (e.g. I bought a 3ware card because they had a good reputation in supporting FreeBSD).
NB: The card you linked to is not a RAID card. It just adds a few SATA III ports.