Two issues: will the larger tire fit your rims, and will they fit between your frame's seat stays and brakes.

Check this out for rims/tire compatibility:
http://www.biketiresdirect.com/a119/...th/article.htm
You can see that a 25 tire fits the narrowest rim in this chart, and I even squeezed a 28 tire onto a 13mm rim.

I bought a $20 set of calipers to measure various things: stems, seatposts, rims. My neighbor said I would be surprised how much I use them, and he was right. I don't think you're going to have a problem with rim width or frame/brake clearances. But if you're concerned, buy or borrow some calipers and measure the inside width of your rims.

But basically, don't worry, get the tires and mount them. I've taken 23's off all my road bikes (although my son likes them on his bikes). The smallest I have is 25's, then 28's, and this week I mounted some 32mm Conti-Contacts on my singlespeed, my favorite size and tire so far.