Probably 1920x1200s...
Really not sure what thunderbolt is going to do yet, but the way to go would be a Matrox DualHead2Go or TripleHead2Go, DP Edition- they'll both support two screens up to 1920x1200 from a single Mini DisplayPort (using adapter to standard display port)- and the triple will support upto 3 screens (albeit not at 1920x1200 each) full Mac compatibility they've tested for the Display Port (not the VGA or DVI) version. Matrox is known for their outstanding graphics cards and expansion modules for multi-display setups, and these are great little boxes that will definitely do the trick!
http://www.matrox.com/graphics/en/products/gxm/dh2go/
edit: 1080 or 1200 for the dell resolutions... could be either one. my bad