I'm hardly an expert on RAID stuff, but by the numbers, I'd say that it would be an iffy proposition.
Firstly, the PCI interface tops out at 133 MB/s sustained data transfer, IIRC, and most modern hard drives have no trouble putting out half of that. Thus, a RAID 0 setup with two drives would basically max things right out, and more drives would be largely redundant, except to provide more storage space (or RAID 1?).
Secondly, the aforementioned 133 MB/s interface would be unable to take proper advantage of SATA II's maximum burst speed of 300 MB/s.
Lastly, simply introducing any RAID controller introduces additional latency into the process, increasing average seek times. This may or may not be an issue. For large file handling, probably not a huge issue, but it's still there. If you have other applications running off the disk(s), they might load a little slower.
Like I said before, I'm honestly no expert, but the numbers seem a little weak from what I can see. I'm assuming there's a reason PCI-E isn't an option for you? Even a PCI-E x1 (250 MB/s) slot would have more potential, I'd expect.