if you have the acr struts still, you're gonna be hard pressed to fit 18's... you'll have to find tires for a normal neon in 18" because of the differance in struts...
a buddy of mine had an OB ACR, tried to put 18's on w/ tires for a normal SRT and they wouldnt fit at all because of the struts.
you're gonna have to be careful of the tire size regardless of whether you get 17's or 18's though
once i get the srt brake setup on my sxt, i'm going to get 18's since i will be able to fit 225/40r18's.....just cant fit em till i get the brakes....