Every time you add new hardware it runs the risk of making your system less reliable, cheap manufacturers and their shoddy drivers being the main reason. But with a good brand, you won't run into many problems.
Yes, the number of pins matters -- a drive with 80 pins won't plug into a card that only has 68 pins. Ordering though is simple enough -- just ask for a U320 80-pin scsi card, voila!
SCSI drives are typically very reliable, long-lasting drives as they're intended (and priced) for enterprise use.
