Hmmm, I was pretty sure the (often affordable, eBayable) 3ware 6410 supported RAID5.

I love and respect the 3ware cards, but in your shoes might just set up mirroring with one of the Arco Duplidisks. What 3Wares (and most RAID cards) *don't* give you is teh ability to swap in pairs of larger disks later on to expand your array size (which is why I like the dumb Duplidisks). I would opine more thootfully on this but I just drove in from the coast (from Portland to Boston in 3 days) and I gotta take a shower and nap.....