I like the Command and Conquer games and many of them are out on budget so theres no need to spend a fortune to see if you like them and if you're on budget as with many games you should be able to download demos to see what you actually like. They are probably the predecessors to games like the Total War series and you might not like them.
Trying the demos saves parting with money and getting something you don't like and gives you an idea of how well they will run.
I agree with the C&C games being good fun - you could also try the likes of World In Conflict and Company of Heroes although these are more based on managing a small number of units rather than the being able to mass produce units after resource gathering (probably should try the demos first to see if they are your thing though)
Out of the ones you've listed I would say Medieval 2 was the best (Empire Earth 3 has been getting some bad reviews)
I also quite liked Warhammer 40,000: Dawn of War and the expansion packs - but I suppose it depends on what kind of setting you're after for your games?
Yes, medieval 2 and rome total war are amazing. Before, I had to settle for age of empires and kings. Just go on youtube and watch a few clips and see how good it is. Medieval 2 requires/makes use of a 2.4ghz cpu or above.
I want to order the medieval 2 total war for £10, but the expansion pack is also £10 and is the upgrade too. I only want to buy one, preferably the expansion pack game. But will I need to have the original game to play the expansion pack?