I can't speak for z68 specifically because I have never used that board. However, you should be able to find more details (ie, in the manual) base on my responses below.
A1:
Personal use of relative. Be specific on exactly what you intend to do and what features you need. Keep in mind that RAID on consumer boards are typically added as "me too" features (ie, they have to keep the same feature set as their competitors to justify the board prices). They are very limited in their functionality and should never be used if you want a longer term, hassle free, reliable setup.
A2:
Windows is very new to the RAID game (as it is with all server features outside exchange), and should not be considered for software RAID solution. You will need to look to other OS which supports ZFS (FreeNAS, OpenIndiana, etc) or Linux md for reliable software RAID with decent performance.
A3:
Yes, you should definitely consider a hardware RAID card. Lots of cards are available pre-owned on ebay within that price range. The IBM M1015 is a great card for consumer use. Stay away from anything made by HighPoint. PCIe 1x will not have sufficient bandwidth for 4 SATAIII drives. Get a 8x card. If you really want performance and a lot of RAID features, consider spending $400+ for a decent RAID card. You get true hardware RAID, onboard cache, etc at that price range. (Note: These features are not necessarily need for home streaming needs but if you have the cash, they do help).
A4:
Those features depend on the RAID card or software RAID you use. Most consumer board RAID (and is suspect the z68 is the same) will not allow you to add drives after building the RAID. Once the array is built, you will only be able to replace failed disks. Adding new disks will involve destroying the existing RAID with your data and creating a new one. To perform these tasks, you will need support for RAID migration (changing from one RAID level to another) and RAID expansion (adding disk to your existing RAID and increase the storage size). I think the M1015 support RAID expansion but not RAID migration (check the manual before you buy) so you will not be able to start with two disk then add a third. Cards in the $400 range should allow this. Both software RAID from other OSes mentioned about supports these features.
A5:
Even though you don't want a separate device, it is advisable to get a dedicated NAS, separate from your gaming rig/every day use PC. Your main PC should not be your centralize storage appliance as well. With that said, there shouldn't be any need to "beef up" your gaming rig for RAID. As stated above, if you are sticking with a single Windows box, all you need is a decent hardware RAID card. That will take care of all the RAID processing. Performance should only be an issue if you're running the game or your OS off the RAID5 array. In that case, get SSD drive(s) for the OS and games.