RAID5 on a budget

[DIALTONE]

I need to look into an external RAID5 setup for my single server office. The server has internal RAID1 but no more free drive bays. Space is not a major concern: 100-120GB would be more than enough for our needs. Basically I'm looking to slap 4 disks into an external box with a hardware controller. How cheaply can this be done and what products might I look at? Thanks in advance. I'd prefer something directly connected to the server as opposed to NAS.