Leaf I think you would find that the 300 2.8 will do a great job it is faster and is incredibly sharp. I would go rent both and do a comparison to see what you think. Out of your above choices I think you are better off with the fixed lenses like the 300 2.8 or the 300 f4 or the 400 f5.6. I think you will find them much sharper with zero barrel effect. Just how fast do you need the lens. I find when shooting people for instance that unless they are full length in the shot you are going to be shooting at f4 or 5 anyway to make sure you carry focus. If you are shooting wildlife that's another thing. One more option would be to add a new software to the slower lenses to add more fall off and the look of a faster lens here is the link.  
        http://www.alienskin.com/bokeh/
 I haven't used it but PDN gave it excellent reviews.

 DPI or Dots Per Inch reflect directly with the end users needs. As earlier stated the web needs much less information for an image than a print so it runs at 72 DPI. The print however needs a higher resolution depending on the publication material and printing style. Example, Newspapers need less than Vanity Fair magazine because of the paper and detail required.
 You can change your DPI in PS if you like but you are as earlier stated just interpolating or letting the computer guess at what extra pixels to add by analyzing the already existing pixels if you want the image to remain the same size.
 There are limits. You cannot take a photo at 72 DPI measuring 7 x 7 inches and interpolate that file to a 300 DPI file at those same 7 x 7 dimensions. If you want to make a bill board from a 72 DPI 5 x 5 inch file you would not be able to take it nearly that far. However if you have a 50 mg. file at 300 DPI then you have enough information for softwares like Genuine Fractals to interpolate that image to almost any reasonable size needed without great loss of information. That being the reason that Macro and Micro sites don't usually offer larger than a 50 mg. file at 300 DPI. They are sufficient for just about any application.
 Which brings me to another I think very important point about the changes in our culture on how we gather information and the price points that are in position at both Macro and Micro levels. Why should a web site that gets as many hits as it's publication version, lets say PDN magazine and PDN website just for example. Why are they offered by the agencies to pay so much less per photo for their web site version than their press version.
 When the internet started and Macro RF was first in place agencies set these smaller prices for small web files because it was extra income but it was in know way a threat to the publicized world that made up the agencies and photographers bread and butter. Now the web quite often gets far more hits than the publication version but still pays much less per image. I think it is time to evaluate the price structure for small web size images to reflect the change in advertising. I'm not talking about making their cost equal to a 50 mg. file not even close. Just consider adjusting for the change in times.
 

 Recently on another thread people started talking about RPI's and there were some interesting misunderstandings that I wanted to point out. RPI is just simply your Return Per Image. The RPI is the gross money earned over a given period of time divided by the number of images you have available for sale. If you have 50 images and you made $500 over the month then that would make your RPI for that month $10, it is that simple. It use to be based off the life of the image, 5-7 years. Then it started being followed by the year, then the quarter and now it's down to monthly RPI. I would like to add that the shorter the time span the less accurate the data on your annual return because some months are much better than others so I think it is best to stick to the yearly average to base your annual income.
 This number is used to help you identify what your gross income is and what you can afford to spend per image to make the profit you are looking for. It is not after all your costs and expenses are included as one person said, it is before those deductions. If you deduct all your overhead and production costs then that is your companies net income, don't forget your salary in there.
 RPI is used less and less in the industry because it doesn't offer you the best feedback for what product is really making you the most money. It really is a blanket overall number that actually falls over time as your collection of images gets larger and older. What is being followed closer is Return Per Shoot. This approach helps you see more clearly if your concept and effort for a particular shoot are paying off or weather they are not. As well as Sell Through Rate. That is how many of your images actual sell in their salable life time.

 Their image upload limits at Istock do play into the picture. SS might have a lower RPI but they allow you to upload a great deal more that easily offsets the returns per month that Istock brings. 50 images at Istock make you x amount but 500 at SS make you a great deal more at the end of the month. Unless you are Exclusive at Istock.
 I wonder if they will pull the plug on upload limits and make other adjustments now Getty might be taking over. Time will tell if we see changes or if they stay the coarse. I hope for all Exclusives at Istock they keep things just the way they are.

 Well as everyone has heard Bruce or Bitter as he is known has left Istock. Good, Bad what are your thoughts. Will this mark a change for the better or the worse. Is it a good time to be an exclusive at Istock or is that a bit worrying. Will Getty take a stronger hand in the operations, will some rules change. Inquiring minds want to know.

NERO