My true point is to get a better IQ, and I wonder if a 1Ds would be able to give me that...
From a purely sensor-based standard, no. Between 1DsIII and 5DII, the base image sensor is the same, resolution and sharpness are the same, and the 5DII actually has very slightly less noise at higher ISOs (resulting from the Digic IV processor, compared to the dual Digic III's in the 1DsIII).
The 1DsIII offers a larger viewfinder with 100% coverage, a faster frame rate (but a slightly shallower buffer), and shorter shutter lag VF blackout times. There are many more customizations (AE adjustment, FEC adjustment, etc.), weather sealing, etc.
Bottom line, if IQ is your main concern and the 5DII is delivering what you need, save your money and skip the 1DsIII.
I agree, the AF and the supurb build as well as long battery live are the main difference between 5D MK II and 1Ds MK III.
I returned my 7D and bought a used 1D MKIII for about the same price. The value of my MK III hasn't dropped over these 2 years, but a used 7D has dropped quite a bit.
In fact, I've been thinking of buying one of the $1164 refurb 7D's(with Canon CLP) and selling my 1D MK III. I would like the extra pixels. However, the biggest issue for me is low light use, and only my 5D MK II can barely get by in the very low light situations.
finally, I saw a used 40d offered locally for a very reasonable price and bought it. I now have it planted in my studio, and have been using a old 22-55mm lens on it. The zoom range is acceptable, but I do manual focus 100% in the studio, and its a miserable lens to manually focus. I'd buy a 17-55 again, but it is overkill for that use of making web photos.
I'm watching Craigslist for a low cost lens like the 18-55mm IS, as well as looking over the Canon refurbs. I've been wanting a 16-35mm for my 5D MK II, so I may just get it and have a lens I can use for both. I've had a 17-40mm L, and the 17-55 was so much better on a 40D.