How do you run them? In software or with a controller card?

I got a 3Ware Escalade 7500-4LP from EBay for ~$150. Little more than I wanted to spend for just a HD controller, but the Escalades are known for very good reliability and performance, especially under Linux. I briefly looked at one of the Promise models that was about $30 cheaper or so, but read about some mixed experiences under Linux.

I'd like to run them with a controller card, but the bandwidth of a normal PCI card (133 mb/s) will not be sufficient.

Well, strictly speaking, the PCI bus might be the limiting factor, but not by much. The average read speed of the SP1604N seems to be around 45 MB/s (depending on where things live on the disk, phase of the moon, etc.) I think the figure for RAID 5 performance is P * (N - 1) where P is the single-drive performance and N is the # of drives. That puts the average throughput of a 4-drive array around 135 MB/s, assuming the RAID5 implementation is perfect. That's right around the speed of a typical PCI bus. For writes, PCI definitely isn't a concern. The only way I can see the PCI bus coming into play is if you're running 5 or more drives, or four really fast 10K drives. FWIW, I'm running my 3Ware on a regular PCI slot and am happy with the performance.

Also, how much free space do you have with four 160 gig drives in a RAID5 array?

RAID 5 eats up the equivalent of one drive for parity, so that leaves you with 480GB to play with (160 * 3.)