On my t2i you can shoot 720p no problems with most cards. However if you want to take advantage of the full hd 25fps video mode you will need at least a class 6. However I would suggest a class 10 just to be safe. If you want cheap look at the transcend class 10 sd cards. 4gb for £6
http://www.amazon.co.uk/Transcend-SDHC- ... 024&sr=8-1
However I have never tried one so could not advise if it will be any good. However it is class 10 so it will not drop below 10mb/s meaning it should be ok.
Otherwise you could get a SanDisk Extreme class 10. 4gb for £16. Yes its more expensive but probably a safer bet.
http://www.amazon.co.uk/SanDisk-Extreme ... 162&sr=1-2
Hope this helped. I may buy a transcend one just because of how cheap it is. If its bad I can use it as a back up.