40mm or 45mm tops I'd guess. fyi - different tire & wheel combos result in different actual width (& height) measurements. meaning a tire might measure wider (or narrower) than labeled depending on brand & model tire & the wheel it's mounted on.
I had some luck last fall (squeezed in some big tires & fenders) but also got surprised this spring (a tire didn't fit at max pressure)
for example, my 40mm Proteks measure 43mm at 90 psi, so I have to ride them at 45 psi which is OK, cuz they have strong sidewalls & don't bulge
as for fenders, that's always a challenge but there are ways to modify fenders if there are issues. I started a cpl threads (in mechanics & hybrids) that might be helpful but they might also bore you & I don't want to hijack your thread. a suggestion, if you want wide, get some 45mm tires & try them out. what's the worst that can happen? you sell them to a friend or a stranger on craigslist?
oh & the OEM wheels are probably just fine
http://www.bikeforums.net/bicycle-me...-fit-bike.html
http://www.bikeforums.net/hybrid-bic...ide-tires.html
http://www.bikeforums.net/hybrid-bic...ek-700x35.html