downsizing?
 

I usually ride a 57 cm. jamis sputnik but i was never really fitted to the bike but it seems to fit well except i've been having problems with my knees. I am in the markey for a road bike and have found one that I like theonly problem is its a 58 cm. and my question is how much of a difference that 1cm. will make??

I probably couldn't feel the difference between two different models of bikes that are 1 cm apart, 2cm on identical bikes yeah, only way to know is to try them out.

It's impossible to say without comparing the complete geometry of the bikes in question. Like Jerseysbest said: ride it and find out.

measure the distance between your crotch (shove a level between your legs or something) and the ground.

multiply it by .883. that number is roughly the distance your saddle should be from the center of the cranks. set it to that and try again. i think most knee problems are due to poor fit. a seat that is too high will cause pain in the back of the knee, too low or too far forward will cause pain in the front.