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 (see the DPT comparison between host-based and SCSI-to-SCSI adapters for why this is the case). Michael Neuffer ( neuffer@kralle.zdv.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 swapability 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 for an initial RAID-5 system of 18 GB, but they are expensive. We are now using a Wetex enclosure ( http://www.wetex.com/), which have the same features as the DPT ones, but are about 1/4 of the cost. The Wetex enclosure system (14 bays) we use is composed of two RAID-5 arrays, with sizes of 45 GB and 63 GB.