you should already have alloys.
remember if you get bigger wheels you will need lower profile tyres to keep the acceleration etc the same - bigger wheels should mean faster top end and slower acceleration due to the amount of distance travelled per revolution, and power to turn the wheels.
the only advise I would give you is to be careful you do not put wheels and tyres which due to the current setup of the car might either scrub the tyres too quickly of put unneccessary load on the steering rack, drive shafts etc. my boss put some new wheels on his BMW and had no end of problems.
I am sure murphwhizz has some real facts rather than gossip though
let me know how you get on I do feel the wheels on the sedona look a bit camp..
you may find that new wheels void the warranty and maybe the insurance