Ben,
I'm assuming you mean DNxHD (not DHxHD)...Avid's codec. If so, then when you transcode from h.264 DSLR footage you'll likely go with DNxHD 145 (as there is no 120). DNxHD 36 is meant for offline editing....though in some cases this may be sufficient. Depends on the footage, the client, the budget, and the post-prodution process.
DNxHD 145 will do great with compositing or color correction needed. Remember you're footage is only ever as good as the original capture format. In your case, a DSLR shooting to the h.264 codec. Currently that is only an 8-bit codec. So even if you transcode to DNxHD 220, you're not going to be adding data or picture quality to your image.
If you setup a RAID0 with 2 or more disks you should be fine on playback with DNxHD. It is a pretty efficient codec (similar to Apple ProRes).
Ryan Holmes
http://www.ryanholmes.me
vimeo.com/ryanholmes
