Congrats, FusionGirlie...
With a 20 (anyone feel free to correct me if I'm wrong, but I'm pretty sure this is right), you can't possibly get a tire thin enough to offset the increased width of the rim, so you're talking about a larger overall diameter.
This has several effects - first, your speedometer will be off. Second, your odometer will be off. Third, running an extremely thin tire means much harder ride quality. Fourth, the increased diameter may or may not physically fit, and even if it does, it may be prone to bumping. Fifth, most likely you're talking about a considerably heavier wheel, unless you plan on spending a lot of money (and even then, with a 20, you're probably going to be somewhat heavier).
Personally, I wouldn't recommend larger than 18's, but I know there's people on here that do have 19's and 20's, so it isn't that they're impossible to do.
Be very careful with the offset of the wheel you get.
