I'm not sure if a 5D Classic is really that much of a "step up" these days. Your D5100 has about 2.5 EV more dynamic range than the 5D; a D7000 has 2.9 EV more dynamic range. The D5100 may actually have a better AF system than the 5D has. I have owned a 5D since it was the hot, new camera...it's been surpassed in the most-critical technical metrics that are important to me, dynamic range and color depth/richness, by most all of the newer, small-sensor Nikons. We need to keep in mind, the 5D is a camera that has been on the market a full eight years now, and which was probably developed with technology that would now be almost a full decade old.
DxOMark - Compare cameras side by side
In a lot of ways, the 5D classic is primitive; on many units, the LED display inside the camera is so dim that it's damned near impossible to see the readouts in the spring and summer, and I am not kidding. The rear LCD is small, and low-detail. It's AF module is weak, and highly central. The 5D has a very good, old sensor, in a really cheap $389-dollar class EOS Elan-type body. In many ways, the 5D classic is a camera for experts with a lot of experience, and who can work "old-school". I made a lot of good images with mine, but the last time I was at the beach with it, I could not see the LED displays inside AT ALL...utterly totally washed out. INVISIBLE in bright light.
I think a case could be made, now, in 2013, that the 5D is not much of a "step up". It kind of depends on several factors. Your question doesn't really give me enough info to say yes or no with any high degree of certainty.