Well, as to the first part of your question, you will obviously get different answers depending on who owns what. I personally think the 240 is way underpowered for spirited driving, and the 320 is certainly worth the premium if you want to spend the $$. It is a tougher call with the 230, but you do still have signficantly more punch with the 320. If you are going automatic 320 becomes a better option (IMO) than a 230. With a manual, the c230 becomes very entertaining to drive, and I think the call becomes tough.
I would also wait for 2005 -- I like the changes. Search the archives for pics on the new design, and decide for yourself. I think the most noticable differences will still be cosmetic (unless you decide it's worth it to splurge for the new AMG V8). You will certainly get better deals on the 2004s in stock.