Most mobos have built in RAID controllers these days, but not all will handle R5.
R5 has speed advantages when reading, but disadvantages when writing. In normal use you won't notice the disadvantage because it is buffered, but if you are going to do a LOT of disk updates then R5 is probably not what you want - get an extra drive and go for R0+1 instead.
You have to set up the RAID array FIRST. Once you have set up your RAID array, it appears to the system as a single drive - in the case of 3x500GB drives in R5 it would appear as a single 1000GB drive. You can then partition it just as you would any other drive.
As far as the RAID controller is concerned, the partition information is simply data. If you get a drive failure then the system will work with just 2 drives (but slower). When you replace the faulty drive it will be rebuilt with the missing data, which includes any information about partitions.
All the drives are needed to support the RAID array. It works like this:
As you can see, the data and parity is spread evenly across the drives so you can't take a drive out to use it somewhere else.
Drive 1 Drive 2 Drive 3
Block 1 A B P
Block 2 P A B
Block 3 B P A
Block 4 A B P
Block 5 P A B
Block 6 B P A
From the sound of it, you aren't absolutely sure what you want from a raid system. If that's the case my advice would be to keep well away from it until you are sure exactly what you want. Remember that changing to/from RAID or the type of RAID destroys all the data on the drives, so it can be a very frustrating process if you get it wrong.