The risk here is making it seem like a "trick." I don't think it is a good idea to use playing cards in a proposal. The trick needs to make sense in the context you are doing it.
One idea is Fire Flight by Peter Loughran -
http://www.penguinmagic.com/p/S9058
OK, the video over emphasizes that your hands are clean, but the idea would work really well if you are using the same engagement ring. You could even write down the things in the past that you want to put behind you and use that paper to wrap the ring.
I like the effect, because it seems to present the appropriate symbols for what you are doing and it is the perfect end to nice candlight dinner.