We have an attached garage that is not heated. In the attic I am planning to put some cellulose insulation over the fiberglass stuff. Once this is done I would like to put some plywood down so we can store stuff up there. Should I leave small spaces between the plywood to keep condensation from forming? Also are there any other reason why I should not do this?

Don't know about condensation however you do need to leave space between the plywood and insulation to let it breathe and not pack down the insulation it needs to be loose and in layers not one big clump. As far as doing it, I would be skeptical for the fact that, heavan forbid, a fire breaks out it would run through there like a rocket on takeoff.

if the final height of your insulation is higher than your joists, then I suggest installing 2 x 3's on top of joists so the plywood will clear the insulation. Be careful not to pinch any wires or cables.No space is needed between the insulation and the plywood but you do not want to compress the insulation- this will decrease its R- factor. Be cautious not to blow insulation over recessed lights that are not rated I/C. You can tell by removing the bulb and looking for an I/c sticker. If there is no sticker then you must have no insulation touching the fixture. Also use just enough screws to fasten the ply from moving. It will be a life saver for future work performed by you or a contractor. I also suggest that you cut the sheets into 2' X 8' sections. It will make it easier to get the sheets into the attic. Good luck.