Okay, guys...support your own!!
http://www.miata.net/garage/tirecalc.html
Any calculation you want to do, and it'll tell you how far off you'll be.
As far as simply tire sizes, the 205/65/15 would be the closest, however hard to find, and we haven't even gotten into the brake clearance issues yet. I went to Tirerack.com, and they have 7 tires in that size which are winter tires. My choice would be the Michelin Alpin tires.
It seems as if the '05s only come with either 16 or 17 inch wheels, while the '04s come with a 15 inch snow option.