Some of the DFI and Asus boards using the NF4-SLI chipset at newegg list RAID5 as supported by the onboard sata controller. Many of the more "deluxe" models add 4 more sata ports bringing the total onboard to 8 however I believe all of them are using the sil3114 controller which is attached off the pci bus giving you a nice bottleneck.
What OS are you planning on running? If linux, take a look at doing software raid5. It's pretty good and flexible providing you do a little research upfront. The sata ports that are part of the nforce chipset itself is not on the PCI bus (Peter or someone please correct me if I'm wrong ).
If you want hardware raid (ie a card that offloads all the RAID5 parity calculations among other things) you will want one of the newer cards that the likes of Areca, 3ware (soon?), and Promise have out that use a x4 or x8 pci express connector.
Gaidin