Overall you may be better served by high capacity standalone tape units if you fall below the expensive jukebox level. HP actually makes very good standalone or internal mount addon units.
But you really need to say what amounts of data and in what time you expect to backup. All commnets about speed assume the selected SCSI interfaces can keep up. LTO-Ultrium format drives are generally the fastest and highest capacity per single tape (up to 400GB uncompressed). LTO drives can record full tapes in as little as 2-6 hours. LTO is the largest selling type now too. http://www.lto.org/newsite/html/format.html
SUPERDLT are not quite as fast and high capacity. IF you are looking to pick up clearance priced drives, a single AIT tape can often back up 60GB/75GB uncompressed to a single tape in about 12 hours.
Remember to calculate Tape costs over drive life. Tapes can cost as much as the drives — especially if you buy from a drive manufacturer. Yes you will want to look for recognizable name brands at a discount mail-order/online store rather than just buy everything from the drive vendor. Most drive vendors cost up to 100% more for tapes and do not change the warranty on the drive.
You should realize that Dell especially lags about 2 generations beind current hardware standards just to make sure that support is painless for them as well. That is reliabilty first and performance/cost a distant second. they do tend to have nice integrated disk/tape packaging though. Nor does Dell very often support upgrading units with newer technology parts — buy a whole new unit.