If you don't mind buying used hardware with little or no warranty, you could buy a Dell Perc 5/i card off eBay for ~$100 or so plus two of these SFF-8484 to 4 x SATA Cables for ~$25. Thus, you get 8 ports and a true hardware RAID controller for around ~$125 or so. Add a battery backup unit for ~$40 and you're set in case of power outages as well up the performance. Not a bad deal considering that a new true PCI-E hardware RAID controller is around $300 (Areca ARC-1210) for 4 ports.
EDIT: As of May 2009, it seems that you can find many Perc 5/i cards with the BBU and PCI bracket for $120 and under. Excellent price!
But those Dell Perc 5/i cards are finicky about motherboards. It will take up PCI-E x16 slot if you don't have a PCI-E x8 slot.
Raptor drives are not enterprise rated drives.. I am not sure I'd use them in RAID 5 for a business. If you need more performance than ES SATA consider SAS..
Really unless you have to build it yourself, a small Dell server is hard to beat price wise and you can get a 3 year 4 hour response time support contract from them..
Edit: A better idea for building yourself: Really rather than RAID 5, do RAID 10.. It takes one more drive, but gives you just about the same reliability with better performance and you don't need a mega dollar RAID controller.