Need some opinions on lens choice

My longest lens is currently my 70-200F4L, however I really want something that will give me around 300 max.

My budget isn't huge, and I find that (for the money I have) my options are basically to get a Sigma 70-300 APO or a 1.4x TC for my L lens...

What do you think would be the best choice based on the above 2 options?

As far as I'm aware, my AF on the 40D will still work with the 1.4x, but maybe my IQ will suffer? But will it suffer more by adding a 'cheap' telephoto to my kit list?

Any advice or opinions much appreciated.

I don't understand, in your equipment list it shows you already have the Sigma 70-300 you mentioned.

I'd have suggested just going ahead and trying the Canon 1.4X II Extender on your 70-200, to see how you like it. I use it occasionally on my 70-200/2.8 IS with satisfactory results. It's never going to beat out a 300mm prime, but is good enough for an occasional "emergency" when I simply don't have a longer lens with me.

If you were to sell off some lenses in your kit that duplicate each other, would it help you toward a 300/4 IS? IMHO, that would be the best choice of all, if possible.

Yep, thanks for noticing that error in my sig..I actually owned the 70-300 APO for a few days on my 400D, but sold it along with the 400D to fund my 40D before I even got it out of the box.