Unless you're using a 500mm lens all the time, ie wildlife/birding/sports are your major subjects, I wouldnt go for the canon 500 f4. It is a really great lens - definately the one if the above applies. If you're a more occasional wildlife/birder, and need the length when you do it, then perhaps go for the sigma 500mm f4.5. This is lighter, relatively small for the focal length, and a lot cheaper. Image quality, while not quite as good as the canon, is more than adequate for most things, and with your 1dsII you will retain autofocus with a 1.4TC, giving a 700mm f8 AF combo. I use just this set-up, and havent been disappointed in it. This shot:
http://www.worldphotographyforum.com...45&ppuser=1512 and this shot
http://www.worldphotographyforum.com...72&ppuser=1512 were taken with the 500mm bare, and these two:
http://www.worldphotographyforum.com...74&ppuser=1512 http://www.worldphotographyforum.com...43&ppuser=1512 with the kenko pro 1.4 TC. All are full frame - not cropped in, but obviously resized for web use. The sigma is handholdable in good light, despite the lack of IS, though as with all long lenses, a good tripod and head combination really make a difference, so if you dont have one (and I'd recommend a really heavy duty one, with a max wt of at least 12kg - mine is rated at 16kg) I would budget for that too.
The £2000 or so that you would save could pay for a new monitor and a 50D to upgrade your back-up body.