Hi I understand that this is has been marked as SOLVED. However, in case I may have missed this mentioned as a solution already to your question, my solution for specific setup has been working great for many years for me. Basically, almost immediately after I purchased the pre-order late 2009 iMac 27 inch model, I add 2 extra, additional 27 inch monitors from Dell which was honestly a beautiful thing - having 3 27 inch monitors, the main as the latest 27 inch iMac and speed wise I NEVER ran into any lag issues. It was cruising so smooth to where I almost at fist felt guilty - as I moved from running dual 27-inch dells for my XP, and WOW what a nightmare IF compared to how well the new iMac quickly adopted IMO.
Anyways, time to get to the point and refer to more your question. Aside from describing the difficulties I had being already 2 years ago and the tech was new, I will simply go over what I chose as my solution in the most optimized way - back in early-mid 2010. I currently have 2 other high end Dell monitors (worth the money!) that fit as nice as I have researched seen since. The sizes of the monitors themselves are symmetrical enough in dimensions and never had any lag / speed issue with the 3rd as I suspected since it is both not designed to have a 3rd plus the device I used to use for connection made me spectical.
For the first (second monitor), I FINALLY found a straight cable display port to minidisplayport that gave a smaller resolution of 1920-1680. The last one was a DVI to USB adapter by Display Link, which capped out at 1680-1200 resolution. Already you can see how much the resolutions varied, which was a big concern for me taking this risk, however turned out fine simply by using iMac as middle monitor as in between the 2 others and ended up looking great, given my setup and seeing other people's reactions the resolution hardly was noticed - in fact it was perfect for the purposes I used each other one for - setting each monitor for specific purposes.
The best part was how I connected my 3rd monitor, which again was quite a risk. Also, I should note on the side I used this third monitor as optional for my macbookpro, and windows 7 that I soon explain. The cable consisted of a DVI->HDMI cable to 3 port HDMI selector (cheap one too), from the selector back to the monitor out for (iMac 3rd monitor) USB->DVI cable to DVI ->displaylink adapter (dvi - female -> USB), then this usb went to my iMac just as all my other USB devices went.
Now I know nowadays that there exist higher resolution monitors and better DVI-USB adapters that are much better in resolution in the least.
Hope this helps,
Mike