Welcome
rockineurope!
have a look through the other sections regarding places in Wales (
pretravel.proboards1.com/index.cgi?board=CCadvice&action=display&n=1&thread=3800&page=1 )and also for other UK destinations as there have been many posts dealing with this in the last few weeks (eg
pretravel.proboards1.com/index.cgi?board=general&action=display&n=1&thread=2952&page=1#1171192619, but there are others). This will give you some ideas of what's possible in the time you have, but while you're looking there, I'm also straight away going to suggest York and Edinburgh and make note that 1 day in Inverness will probably be spent on a trip to Loch Ness and the Highlands (am I correct?) so you won't actually see Inverness. As long as you're not bothered by that then keep it at one day, but the Highlands are just gorgeous. Going to Ireland would always be encouraged so if you can make it work, look at the above post or
pretravel.proboards1.com/index.cgi?board=general&action=display&n=1&thread=2949&page=1 for ideas.
As for the itinerary and time in cities, I'll make some notes, but I'm assuming this isn't the route, just a list of cities you want to visit:
Paris - 1 week will allow for day trips (Giverny, Normandy o'night, Mont San Michele or even Lyon)
Berlin - good number of days, also looking at your other cities, you could include Dresden on the way from Berlin to Prague by train and even Munich as it's near to Salzburg and fits in with train travel.
Prague - 4 days will allow for a day trip to Kutna Hora or a trip to Cesky Krumlov
Austria - Salzburg is good, but look at including Vienna as it's on the line of travel and is an amazing city (for 3 days) and Innsbruck is also in a possible line of travel (eg Munich, Innsbruck, Venice).
France - look at keeping these cities in order by starting in Paris and working your way across, otherwise you're zig-zagging a little unnecessarily.
Aix-en-Provence - gorgeous, maybe 2 days
Arles - again, gorgeous and 2-3 days is fine if you want to explore and relax
Marseilles - not the nicest of cites, great Bouillabaisse
though! - so look maybe at
pretravel.proboards1.com/index.cgi?board=CCadvice&action=display&n=1&thread=3790&page=1 for some other ideas.
Italy - so many threads on destinations for here! But for the ones you have:
Venice - 2 days
Florence - 4 days (5 will allow a full day to Siena as well as a half-day to Pisa)
Pisa - see above (it's only 1hr from Florence by train)
Rome (5 days min will allow for a day trip to Pompei)
Sicily - 2 day is nowhere near enough time on this island, so either skip it and save it for later, or add more time.
You could also look at smaller places like Cinque Terre, Siena, Verona and Lake Como, depending on your interests.
Athens- 3 days if you're not going to an island (you'll need more time if you are) and fly from say Rome to Athens to save time.
Once you put together the next version of your itinerary, let us know and you'll get some more advice for sure! Oh and it's ueber (or über) not ubber ;D but either way, we'll help!