If you can't answer those questions, it makes it tough to make a recommendation. You are right that the D5k is missing a few features. Going up to the D90 doesn't get you a whole lot more, going to the 50d, gets you a a little more, mostly in terms of speed and build quality. Have you missed shots or had a hard time getting a shot because of the D5k?? Honestly, unless you are shooting sports or action, or a regularly abusing the camera, you probably don't need the 50d. Wanting a camera is one thing, but do you really need it? A flash, better lenses, or investing in a tripod will do more to help you get better images than a new body. Especially if you are just going to pair a nicer body with a superzoom. Also understand, neither will get you better images (maybe marginally so, at least not enough to be immediately noticeable). I don't think getting a camera because it is more professional looking is necessarily the right decision. There is something to be said to having a lightweight camera, and again for me, my D5k is my first choice for daily use over my D300.
If you've got money to burn, then really for a few bucks more, the D300(s) is probably a better choice than the 50d, simply because you will be able to swap lenses , and the control system is the same. Why keep the d5k as a backup if you won't be able to use your canon lenses (if the 50d fails) on your backup. And again, going back and forth between systems is not easy.