225-45-18 should be nearly the same OD and keep loading similar, not sure about 215-45, that would be too short(I am assuming your car has 225-50-17 on there now, like maximas do).
I would check into the bridgestone s-03's. call up grant, they run about 200 a peice in that size.
if you want to get away cheap:
http://www.tirerack.com/tires/...1=yes
check those out, not great overall, but they're cheaper than anything else in that size.-chet