d_stickney,
If you want to be competitive keep the same size wheels. If you don't, you will end up getting bumped to another class. Right now, you just need to get seat time, get a rule book and observe the more experienced autocrossers. This will help your driving and give you a chance to observe what is working and what is not working.
If you watch the "for sale" part of this forum, you will see a set of wheels that appeals to you. Sometimes they will have tires as well. If fact I think someone is selling a set of MSP wheels there now. If you get something other than the stock wheels, make sure that the wheel's offset is close to that of the stock wheels or you will be running in a different class.
As for tires, in the Open stock classes you can use any DOT tire that you can get on the car. In the DOT "R" tires, usually, there are two sizes to pick from for the MSP, 205/40-17 or 225/45-17. Despite all the standardized sizes, not all tire of the same size have the same dimensions. This is important since some of the 225/45-17 will fit on our wheels and not rub while others will rub. If you do a search, you can see which are which at least for those that people have tried. The Yokohama A032 is considered a DOT "R" tire.
Another thing you might want to check on is a street tire class. Our region has a street tire class requiring street tires whose wear rating is 140 or above. This is like the rule for STS except it is applied to all classes. These entrants then have their best time mulitiplied by the car's class' PAX factor to determine the order of finish. In my case, I run a DS MSP on street tires. So my times are multiplied by 0.794 to get my time. At the last event it went like this, 56.666 x 0.794 = 44.993.
This is the way I have been running since I have gotten lazy in my old age.
Other sources for wheels and tires include:
http://www.tirerack.com/
http://www.discounttiredirect.com/direct/home.jsp
I hope this helps.