Will you really be saving money? I'm thinking of going the expansion card route merely because it is actually less expensive. Case in point: An Asus P2BD usually goes for around $289 while it's SCSI-equipped brother, the P2BD-S costs $550 (your actual prices may vary). Granted, P2BD-S's SCSI is ancient in computer terms, I'll bet that the differences on the newer boards are similar. A SCSI controller can be found for as little as $175 and can come with everything you need to connect Ultra-SCSI 160 devices (internal and external) and legacy devices (internal and external)all at the same time.
Go to Tekram's web site and look at the DC-390U3W and look for it at pricewatch.com. It seems to be a pretty good deal. I have an earlier posting on the forum asking for feedback on Tekram controllers.