I am a slow-ish, newbie racer and recently upgraded from a 20-21 lb steel Bianchi to a sub 16 lb carbon fiber Look 585 for road racing and the difference is astounding. The stiffness of the Look frame gives me noticeably more "snap" for quick accelerations and the lightness is very noticeable on climbs. I can't imagine racing the Bianchi now after racing on the CF Look frame.

My question is this: I had planned to race cross this upcoming season on a steel Gunnar Crosshairs that I have (currently used as a back up roadie and a commuting bike).

I like the Gunnar a lot (the fork is buttery smooth and glides over bumps with ease), but it feels like a heavy pig to me now. Since I am an old guy with some disposable income, I am contemplating getting a Ridley X-Night or some other lighter cross frame. The Gunnar maybe comes in at around 20 lbs.

So how much does weight matter in cross? Should I stick with the Gunnar or go for something lighter?

I'd race the Gunnar a few times and see how you like cross. As for weight, I dunno, you'll always find guys that tell you it's all about the engine and you shouldn't worry about bike weight, but I had the opportunity to shed a few pounds from my JTS last season for small money (CF fork and then later better wheels). Each change was ~1lb and I definitely think I could tell the difference. I don't think it was placebo effect.

I'd say that weight savings in cross is even more beneficial than in road racing without of course compromising the structure of the bike and it's parts because they take an awful beating. If you have the money I'd go for it. I'd still say race a few on the Gunnar too though, that's a nice bike in it's own right and 20LBS is a light bike. But after 45 minutes of racing and you have to carry that bike one more time you would be happy with a nice light bike for sure. My Trek weighs in at 20.5 FWIW.

I don't think that weight makes much difference in cross. There's not much elevation change and you only have a few opportunities to pick up and carry your bike. In my opinion, the folks that are most successful have a good level of fitness and an excellent level of technical/bike handling skills. Fyi - I race a surly crosscheck set up singlespeed in C and B races.

On the other hand, there's nothing wrong with upgrading. :)