Given all these options, if you're looking for a RAID solution, you need to think carefully about what you want. Depending on what you want to do, and which RAID level you wish to use, some cards may be better than others. SCSI-to-SCSI adapters may not be as good as host-based adapters, for example. Michael Neuffer ( neuffer@uni-mainz.de), the author of the EATA-DMA driver, has a nice discussion about this on his Linux High Performance SCSI and RAID page.
The enclosure type affects the hot swap-ability of the drive, the warning systems (i.e., whether there will be indication of failure, and whether you will know which drive has failed), and what kind of treatment your drive receives (for example, redundant cooling and power supplies). We used the DPT supplied enclosures which work extremely well, but they are expensive.