[sassan]

Make sure you don't fall for 75-300 IS that has several problems. For a little higher price 70-300 IS is fine.

If budget is tight, look into Non IS ver that in USA is about $ 150, IMO one of the best priced for quality canon lenses.

If you are more serious about the quality of your images and have larger flexibility with the budget, you may want to look into one of the 4 sister "L " class (Best quality glass canon ever made) in the 70-200mm range. You have F 4 non IS in lowest price spectrum, upto F 2.8 IS, highest price. They are the best money can buy for you. You will never regret to have spend your money for any one in this class (Vice versa you may feel very much regret otherwise) and as these lenses do keep their value very well, you can always sell them on ebay for almost what you paid for them as long as you have not damaged your lens.

Adding a 1.4 Mk II extender, you are at almost 300mm equivalent without any quality loss. 2x is said to decrease the quality a bit (I am yet to see that in my own experience) and gives you even higher reach out.