I doubt I'd go with that controller for a number of reasons. About the only thing going for it is cost.
The cable attachments are external and that would require passing the cables back into the case, normally a bad idea. Next, There are limited reviews of the card, mostly on a Mac OS and user reviews on Newegg were about half nonfunctional. No reviews on storagereview.com yet either. It might be a decent entry level RAID controller or it could be a complete dog.
From a performance standpoint, RAID-5 is going to be painful. It almost always is...especially from a write standpoint. Parity calculations on a soft RAID processor and no onboard RAM are going to be slow. Your better bet would be either two drives in RAID-1 or 4 drives in RAID0+1 on this or a similar controller. The performance will be better than RAID-5 and for the RAID-1 option, the two existing ports on your MB will do the job with less latency penalty than a PCI-X card.
PCI-X will not add anything form a performance standpoint, since your board has only PCI 2.x slots. Using the onboard drive controller with direct access to the southbridge will probably give a bit better results on burst data than using the PCI-X card.
Reliability is another major concern and I'd think about products with a longer backtrail as a better potential option. Redundancy is fine, but if the controller dies, it could take both disks with it into oblivion.
The 250GB drives are fine, but you didn't mention backup strategy. RAID is not a backup. Backing up to DVD would probably be a decent solution.
For alternative cards, I'd probably recommend either this one for a 2 port RAID-1:
http://www.newegg.com/Product/CustratingReview.asp?item=N82E16816102060
or either of these for a RAID 0+1:
http://www.newegg.com/Product/CustratingReview.asp?item=N82E16816102080
http://www.newegg.com/Product/CustratingReview.asp?item=N82E16816115030
The second link, Highpoint, does support RAID-5, but I'd expect some performance hits. Both of the 4 port options are a bit more expensive, in the 130.00 range.
The bottom line is that this is an engineering tradeoff. You have three criteria, cheap, fast and reliable. You get to pick two...
Please remember, even with RAID-5, RAID is not a backup. I've seen too many arrays lost without backups.
