I have a 2004 Specialized Roubaix (53/42 12-25) and I want to tweak my gearing a little. Everyone says it's cheaper to change the cassette than the chainrings. If I'm going to replace the cassette, I might as well go to a 10 cog if I can. How do I know if my frame will have enough room for the extra cog? Secondly (I have just enough knowledge to be very dangerous ) from everything I've read, the cassettes for a 9 and a 10 are similar in size, they just made the chain narrower, so I assume I'd need a new chain, but the bigger question is whether I'd have to change out the chainrings too, would the teeth be too 'thick' for the new chain? If so, would it be cheaper to make my adjustments by changing the chainrings and leaving the 9 cassette right where it is? Any issue with any other parts going from 9 to 10? Shifters, derailleur, anything?

I'd appreciate any thoughts or info!