Read performance of RAID5 vs. RAID0 should be pretty similar, and better than a single drive, unless the RAID5 implementation is quite poor. I don't really have all that much statistical info on those modes under Linux, but thye've come up with some odd stuff before, so I wouldn't put it past them. (I'm talking solely SW RAID here; HW RAID should be pretty fast, I'd imagine, for all bets, although I'm not familiar with these consumer RAID things, although relative speed might vary quite a bit between different modes.)

Writing, on the other hand, is way slow on RAID5.