Seriously, I suppose they have a point though. Not many members have tried 17s. Top_Fuel is on the right track - post up some specifics on what you have in mind, and we'll respond with a pages-long debate on it.
For rims, the bolt circle diameter is 100mm with a 56.1 bore. Stock rims are offset around 46 (I think...)
Have a look at these threads for inspiration:
Wheels, Tires, Brakes & Suspension Sticky Index (look here for good info)
Wheels: What Else Fits the Mirage? (With photos)
Choosing Fuel Efficient 15” Wheels and Tires
And before you buy anything, read this:
Fuel Efficient 15-Inch Wheels and Tires
And be careful modifying - it'll empty your wallet.