Upgrading Canon telephoto lens - need advice

I own the following Canon lens
10-22, 24-105L, and 70-300, along with my Canon 40D

This question is regarding the 70-300 mm lens which I would like to upgrade. Suggestions would be greatly appreciated.

The 70-300 is a great lens and has been giving me some good quality images with great detail (see -
http://www.naturephotographers.net/imagecritique/ic.cgi?a=vp&pr=137849&CGISESSID=7fbf058cc039648eec7602cdafc0bcac&u=25681
http://www.naturephotographers.net/imagecritique/ic.cgi?a=vp&pr=137804&CGISESSID=7fbf058cc039648eec7602cdafc0bcac&u=25681
)

However, I am not getting the reach I want, and I need to work pretty hard to get close to subjects. Cropping the image (esp. at high ISO settings) is not working out for me.

My price range for the upgrade is < $2000 and ideally <$1500.

Here are the options that I am considering. Can someone please help me with what might be best -

a) replace 70-300 with 100-400L. What concerns me most is the image quality of the 100-400. Also I am thinking of coupling the 100-400 with a 1.4x TC or a 2x TC - but AF will not work with these. The versatility - of 100-400 is really helpful, but not too sure of image quality, esp. if coupled with 1.4x or 2x.

b) Keep the 70-300. Get a 400 f5.6 L. (AF will still not work with 1.4x or 2x TC). Lens doesn't have IS.

c) Keep the 70-300. Get a 300 f4 L IS. AF might work with 1.4x. However I haven't helped myself with reach.

As you can see - not many options given my price range. Any suggestions as to what I should do. I haven't looked at Sigma or other lenses. Does anyone have a recommendation from those lens ?

Advice greatly appreciated.

Gaurav, I have the 100-400L (but later upgraded to the 500 f/4). IQ in the zoom is quite good IMO and several folks in Avian use it with excellent results (moderator Nick Saunders for example). I still use the 100-400 at times when flexibility is paramount and it does a nice job. I tried manually focusing with the 100-400 coupled with a 1.4 tc and gave it up almost instantly.

I don't own any of the lenses you are considering, but I have researched them all. Here is what I've learned:

The 100-400 is a good choice - an L lens with a bit greater zoom capacity - very versatile. A lot of people abolutely love this lens. Some folks have complained about its sharpness, especially at the long end. Coupling with a teleconverter may degrade image quality.

The Canon 300 has an excellent reputation, and with the 1.4xTC will give you 420mm. With the 2.0xTC you'll get 600mm, but will you lose AF at f8. Both of these teleconverters perform excellently on my 300 f2.8 and I think the same is true for the 300 f4.

The 400 f5.6 has a very good reputation for sharpness, It is prized as a flight lens, even though it doesn't have IS. I used one once - it is surprisingly small and lightweight.

I moved up from a 70-300 to the 300 f2.8 L IS with both the 1.4x and 2x TCs. Despite its price and heft, I haven't regretted it.

Tough choice. Good luck.

Thanks for the replies so far. While researching, I also came across Tamron 200-500 mm. Any feedback on this lens? Given the price, I am assuming that the IQ is lower for this lens than the above canon lens (70-300, 100-400, etc.)

Gaurav - I have owned the Canon 300mm f/4.0L IS lens for 3 years. It has excellent image quality, works very well with a 1.4TC (I don't have a 2.0TC) and has beautiful bokeh. However, for some time I have been contemplating the purchase of the 100-400mm lens primarily because I have missed too many fauna/avian shots where I needed the flexibility of a zoom as opposed to a fixed focal length. I mention this because I think in consideration of your new lens, you really need to assess how much this flexibility is important to you. Since you are now used to a zoom telephoto, it may be hard to switch to a fixed focal length. But if you know that all of you shots are of birds way off in the distance, zoom lens flexibility may not be important.

I use the Tamron 200-500mm. It is an ok lens. It isn't the fastest, and I miss having IS. I've taken some great photos with the lens, and I'd say it is pretty good for the price. Photozone www.photozone.de gave the lens some pretty good marks.

Thanks all. I traded in my 70-300 for a new 100-400 eventually.