Bandwidth of what? You're confusing the interface (SATA 3Gbps) with the speed of the drive. IF each drive can do no more than 125MB/s, then two disks on a PCIe x1 would be fast enough - since this offers 250MB/s for PCIe x1 gen1 and 500MB/s for PCIe x1 gen2.
Besides i don't think you can use the PATA disks on the same RAID array. If you do, 4 devices on PCIe x1 is indeed a bottleneck. But likely you'll face other problems that cause you to not saturate the PCI-express interface.
The real question is how efficient the RAID-drivers are; and i can tell you Windows software RAID does a better job. So if you don't need boot support; i'd recommend software RAID0 instead.