The 70-300 IS USM is an absolutely cracking lens or if you want longer the 100-400L, in addition to his two I have also had the Tamron 70-300 and after a week changed it as it was useless, the colours and IQ were shocking.
I appreciate money is tight, but you can get some good deals on a second hand 70-200 F4 L if you can stretch or save a bit longer, its a great lens.