What amp do you have? What iron do you have in it? What iron are you looking at? What other amp(s) are you considering if you go down the replacement path?
To answer your questions, it's going to vary how big of an upgrade it is. If we're talking Speco to BH or MQ, yah, that's pretty huge. Without knowing specifics, it's hard to guess how big of a change it will make. Bigger than caps is a reasonable expectation, most things considered.