"I am comtemplating buying a new Dell desktop and was wondering if it's a good idea to just buy a new monitor with it or save some money and use my old one"
It really depends mostly on the age and condition of the old monitor. You do need to be sure it is a PC type, with Plug and Play capability, or at minimum, will start and display with the minimal Windows default VGA driver, so you can load a dedicated driver if necessary. Generally speaking if your monitor is more then a few years old, and not a high end unit to begin with, you are probably better off taking the opportunity to get a new one with your system, particularly if they are offering monitor upgrades with the package you are considering. New CRT monitors of the entry and mid level are not very expensive to begin with, and since you are spending all the money on a new system you may want to start all new. However if you monitor is is less then a few years old, and still works good, and is a Plug and Play type, I guess you could stick with it. If that is your choice, then re look at your system package you are considering and look for offers for either discount on whole system, memory upgrades, or rebates instead. Generally, Dell packages offer somewhere between a 10-15 percent discount or upgrade value, so be sure to take advantage of that, blowing off a monitor upgrade in a package can be valued at about 5 percent (depending on the total system cost of course). Just some thoughts.
Dell Latitude 6430u, w/USB 3.0 dock, Venue 8 Pro Tablet