Don't do RAID-5, stick with RAID-MIRROR... with a parity (RAID-5) setup, you can still only afford losing a single disk, and the rebuild operation when that disk fails is intensive, making it likely for a second disk to fail during the rebuild. With mirroring, the rebuild operation is a bit less intensive (although you still have to read the entire of the other disk, for a 2-disk mirror).
The Pro editions of windows support software natively, and you might as well stick with that rather than buying a cheap RAID card. If you want to buy a RAID card, you'd have to go for one of those 3ware/whatever with onboard RAM and battery-backup to get any real advantage over full software raid.
The entry-level promise cards (as well as onboard raid on motherboards) don't usually offer any advantage, and at most an XOR engine to relieve the CPU a bit (and in some cases those XOR engines are so slow they end up impairing disk speed).