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!

Going from 9-speed to 10-speed also require a new, 10-speed shifter - unless you're using friction shifting.
But no issue with frame width.

I'm surprised that you have a 42 inner chainring. If the crankset has a 130 BCD the first step you should take is replace the 42 with a 39. You could also change the cassette to a 9-speed 12-27 or 13-30.
What crankset do you have?
The shifters, cassette, and chain must be compatible in "speeds".
Another possibility is replacing the crankset with a 110 BCD 50/36.

the crankset is stock - FSA Carbon Pro, five arm, 3 Ply carbon arms.

If I have this right, the cassette may be cheaper, but only if you are staying with the same # of cogs, if not, I'm going to have to change the shifters and the chain also, and then the chainring would be cheaper to change. The other option I was considering was going to the newer standard 50/34 and leaving the cassette, but not sure about a few things - Do you have to change the crank arms also, or can you just replace the rings (everywhere I've seen them advertised as a set, not sure if they come seperately). Anything that would make me change the bottom bracket?

Thanks for the help, I really appreciate it.

50/34 cranksets are 110 BCD meaning new cranks, chainrings, and probably new bottom bracket. Your present crankset is probably 130 BCD.

Al - Thanks for the info, I think I'm going to start by changing the inner 42 to a 39. It looks like a normal hex key on the front, but the back looks funny - - - Do you need a special tool to switch rings, or can I do that at home?

On the back it's usually just a slot. Anything reasonably stiff that can be fitted into that slot will do in a pinch. Although I will admit that the proper tool is easier to use.

Thanks dabac, appreciate it.