'upgrade'

I'm looking to upgrade my 2004 Trek 1200 and have so far come across:

2012 Trek 2.1 - discounted to £900 from my LBS
2011 Cannondale CAAD 8 105 - £815
2012 Focus Variado 2.0 - £694

I'd probably cope with Tiagra (thats what's on my current bike) but most of the Tiagra spec bikes don't seem much cheaper.

Main reasons for upgrade are: existing bike is a 58cm and is a bit too long for me (TT is 575mm and I have to slide the saddle all the way forward) and everything on it is getting a bit shabby. (it was bought SH off Ebay in 2008)

I currently have the standard Ritchey Pro DS that came with my Boardman Team Carbon. I am thinking about upgrading my wheels to perhaps the Mavic Cosmic Carbone SL's.

Is it worth upgrading? If so is this the best upgrade? They cost around £760.

Cheers!

Hello one and all...
1st post for me so bear with me!

I am rolling around on a Giant Defy 3 2010 which comes with 'Giant Sport Road CR, Double Wall' rims - ones I believe to be pretty good for standard wheels on a lower end bike.

My questions to you all are:

1. If I am going to upgrade anything, is is a good idea to upgrade the wheels first?

2. On a budget of £200 max can I get something that will be notably superior/ marked improvement to the standard wheels on the bike?