The prices you list look the f/4 IS vs. non-IS prices to me (and to photoprice's price history graphs). It's looking like it might be $50 less than usual about now, not $500 less, so there's no need to rush into this.

The non-IS is around similar sharpness to the 24-105mm (at 100, going by Bryan's ISO charts). The IS gives IS, but also a newer design with a sharper image. It may even be the first of Canon's recent lineup of super sharp zoom lenses. If you find yourself cropping a lot, or wanting crisp clean high-contrast edges, for text, or something like blue-merle dog fur you may want to save your pennies for the IS version. If you want more background blur for subject isolation, and don't want a prime like the fantastic 135mm f/2 or 200mm f/2.8, then you may want to save your pennies for the f/2.8. If you won the lottery, perhaps the f/2.8 IS II.

If you do consider one of the other lenses, you'll also have to throw the 70-300mm f/4-5.6L in the mix, for a little bit more. The f/5.6 aperture is only past the 200mm limit of the other lenses. The only downside is f/5.0 from 155-200mm+. Under 155mm you're looking at f/4 or 4.5, which is pretty much the same aperture as the 70-200mm f/4L. If you can deal with being 2/3s of a stop slower as you near 200mm, you can get a bonus 100mm.

If the $700 mark is about the limit of your spending and you don't need IS or modern uber-sharpness, regular sharp is good enough for you, the 70-200 f/4L USM will make you very happy.