next question:

if i get a set of 18" rims, what width rim can i go and what size tire?

i know the bolt pattern is a 5 x 115 and with the 18" rims i have seen recommended 225/45'* but the rim width has varied a bit.

what is the biggest fattest tire rim package we can do while keeping the odometer and mph accurate?

I'm running 245 55 right now, a lot more meat on the tire. I'd say 235/245 series are a good choice.

dont go any wider than 18x8" and dont go any wider than a 255 tire...I would suggest an 18x7.5 with either a 235-50-18 or 245-45-18....235-50 is what came on GXPs and you can get a BFG G force KDWS for pretty cheap compared to anything in a 245-45-18 size (which is slightly wider but you wouldnt notice a real gain by doing it)

i found this cool tire size calculator

http://www.miata.net/garage/tirecalc.html

and here is what it told me about finding an equivalent size for the tire diameter:
tire size TW SW rim width tread width OD
235/55R17 98H RRBL 89205 6.5 - 8.5 9.4 on 7.5 27.2
235/50ZR18 97W BSW 64168 6.5 - 8.5 9.6 on 8.0 27.2
255/45ZR18 99Y BSW 84296 8.0 - 9.5 10.1 on 8.5 27.0
285-295/40 18 27.0-27.3
335/35 18 27.2

the first three are actual tires the last two are using the calculator.
i don't think i will find a tire in the last two sizes i don't think they would fit on an 8" rim anyway. so, it looks like the biggest i can go is the 45'*. good info i think.

255/45ZR18 99Y BSW 84296 8.0

That'* what I would go with on an 18 x 8" wheel........chrome of course.