Multiple operating systems/partitions on a hardware RAID array

Filesystems :

Seeing as you say you want it to be pretty simple, my suggestion is that it would probably be best to stay away from file-systems that are relatively new and still developing features like btrfs. I'd recommend one of the filesystems that are more mature and stable, such as ext4. Some that you may wish to check out :

Ext4 - Probably the most popular Linux file system. It's file limits are far more than you'd likely need, and it has a whole bunch of performnce improvements over Ext2 and 3 systems. It can only be grown offline.
XFS - been around for a long time and the second line on it's Wikipedia page is that it is particularly proficient at large files. It also supports online growing for if you add another hard drive later.
ReiserFS and possibly it's newer revision Reiser4 - also been around for a long time, and offers many of the same features, however future development may be somewhat up in the air after it's developing company's owner was convicted of murder(!). It is currently still actively developed and supported by the Namesys company though and most likely this company will be sold to someone else soon.

Also check out the wikipedia page comparing filesystems.

Hardware vs Software :

If you have the money, I'd say go with hardware RAID every time. When I say hardware, I mean a dedicated RAID card that supports RAID5/6. RAID that comes "built in" to the motherboard, is not real hardware RAID (except for some special server motherboards). If the budget is tight though, and if the CPU on the server the RAID array is going to be installed on isn't doing anything else anyway, then you may be able to achieve almost equivalent performance from a software RAID implementation. Also hardware RAID is more likely to better support hot-plugging than a standard PC motherboard (although in both cases you'd usually also want a SATA drive backplane if you want to hotplug devices).

Both software RAID in Linux, and Hardware RAID present one large "drive" to the filesystem. Hardware RAID implementations are usually completely transparent to Linux as all of it's configuration is usually done in it's BIOS, accessed as the PC starts, but most Hardware RAID cards will also have Linux tools you can use whilst it is online.

You can run multiple arrays on the card. Arrays typically configured in BIOS. A driver will likley be required and supplied by Intel