just keep in mind the wider the rims, the more it will stick out of the guards with the same offset, and you need to consider whether the wide rims will scrub the inner suspension component too. just use the rim size calculator here to check the combination that you want whether you will have any issues, and check your existing rims now and see how much clearance you got inwards towards suspension and outwards up to the guard.
http://www.1010tires.com/WheelOffsetCalculator.asp
best to keep the front rims less wider than the rear since the wider the front rims, it might scrape the guard when turning.
but if you don't lower the car it shouldn't be too much issue, otherwise you may shave the tyre edges, potentially making the tyre weaker,
and your guard could be dented from the scrubbing. It happened on my old R33 even on 17x8" at the front, cos it was lowered 1" at the front.
happened during hard sweeping corner on the race track, shaved the tyre and dented the guard lip at the point of tyre contact...
the rear guard is wider so it can accommodate wider wheels at the back, so no worries.
PS: from previous posts... I will keep this link handy too to calculate tyre size http://www.miata.net/garage/tirecalc.html
Armed with these 2 links / calculators, it will answer all your questions...