If you only want to choose from this provided list ...then i think you should go with the hd 5770...it's the best among the list with dx 11 support...will be very good for you and the games that you have mentioned.,,,
2x 5750 is a well known budget performance setup. It can compete very well with a $300 5850, and blow away a single 5770. Of those choices, that would be the one I would lean towards.
5570 and GT240 are not gaming cards, they are top tier HTPC cards, and I don't believe that the 240 even supports multi-card setups. 3870x2s are pretty old tech, and the power consumption alone would dissuade me from going that route.
Budget wise, you will pay a huge premium for triple PCI-E motherboards, with the cheapest running a x4 slot, which will cripple the performance potential of running 3 cards.
a third option would be to get a single PCI-E slot motherboard (save a few bucks) and max out the GPu budget on a single 460 768mb model, or a 5830. These cards can be found for less than 200 with a bit of dedication.
All of the boards I am looking at are (2) PCiE 2.0 x16 under 200 (for my 200 is the magic number I can carve out of my budget to get this system (if you missed my last post it is for my kid he dr. peppered his last one and when he works with my Wife he makes 200 a month but school is starting soon. I told him I would match what he puts towards the computer which give him a MAX budget of 900 for eveything including shipping)
the Q6600 would be on a X48 Board the X4 would be on a 890X chipset (the one with out onboard video)