I looked up your DVR you can only record in half D1. See page 31 in the manual.http://www.specotech.com/DVR/TNfamily/S ... r_2121.pdf
If you want to know if your existing cameras are sufficient you can try this trick. Unplug a camera from the dvr and get a bnc to rca adapter, then plug that camera in a television with a external video input. The picture you see here will be the best the camera will put out. Can you read plates like you want, can you id people walking in the door, etc.
For a new DVR many people like the Dahua dvr as best bang for the buck. It is rebranded under many names like Q-see and also available on Ebay.