Dell Optiplex PSUs are notoriously underpowered, and depending on the model, a drop in replacement can be tricky if it goes south.
I always advocate taking the safe route: leave your PC's PSU to do it's job for the computer. Don't stress it by putting outside loads like lights, cabinet fans, etc on it, and at the same time you remove the (albeit slim) chance of frying your PC/PSU should something short out.
Chances are you have orphan wall warts around that could power the coin lights, which I presume are 12V? The fluorescent in the marquee though, is that 120V, or are you talking about a CCFL case light type of thing?