I am running 225/50-15s on my stock wheels, and that is a very noticeable improvement over the 215/60-15s I have tried...
The tires I'm currently running are Toyo Proxes, and they are cheap too
I had a lot of understeer with the other tires I've tried (whoever said Dunlop A2 Sports were 'performance' tires anyway - ::Smack:: Tire rack
)