25mm tires

S-Works Turbo 24mm

A general guideline is that the tire width should be between 1.45/2.0 x the inner rim width.

if your rims are 20mm wide on the outside they'll probably be around 16.5 on the inside so should be able to handle 24-33mm tyres. (16.5x1.45)-(16.5x2)
http://sheldonbrown.com/tire-sizing.html

I'm assuming that the 20mm measurement of your rim is the outside dimension from brake surface to brake surface. Assuming this, the internal size of your rim will probably be 14mm.

25mm tyres will be absolutely fine. I've been running them on 13, 14 & 15mm rims for years.

Should be fine.

But you need to try them for yourself, that's the only way to know.

I read somewhere that if you have narrow rims like yours, the advantage of 25mm tires is somewhat negated. With that said, I too have narrow rims (Mavic) and run 25mm Vredestein Fortezzo tires and they seem to roll smoother and the ride is a bit more comfortable.

I currently have 25's on the back and 23's on the front due to some advice and have to say it feels more stable and smoother.

I have just gone from 23mm to 25mm and certainly see the difference in ride comfort don't feel the bumps in the road as much and then plenty of reviews to say less rolling resistance with bigger tyres which could be another positive. It's a must from me if they fit to your frame.