Don't worry about changing your wheels. If your frame has the clearance, buy some wider tires and spoon 'em on.

But first: Try it with your current tires. I did all kinds of off-roading with my 28 road tires. Just don't get too aggressive and you might be fine.

If you still find you need something more, you can go to a wider road tire like this: http://www.jensonusa.com/store/produ...Ride+Tire.aspx (32s or 35s)

Or you can go to a cross tire if you really wanna get dirty: http://www.jensonusa.com/store/produ...700C+Tire.aspx

Enjoy.