I run an eyefinity setup and can give you a quick run down. You will see a difference between 1920 and 1600 even on a smaller screen, but bigger screens tend to result in a lot less eyestrain. With regards to the DVI setup, with normal cards, you would need an active adapter for a DP --> DVI connection for one monitor. In this case though with the FLEX, you shouldn't need to worry. The only thing you will need is a DVI to HDMI cable, which can be found for about £5 from most of your favourite computer etailers.
However if you are planning to game at eyefinity resolutions (5760*1080), a 5770 will not suffice.
To put it simply, unless the games you play are ancient they won't run at decent frame rates in eyefinity. To gain extra desktop real estate, it is worth it, but for gaming you need to be looking at a 5870. Another thing to note is that with an eyefinity display instead of the windows filling just one screen when you maximise them, they will fill all three. This means you will get used to resizing windows a lot, and you will need to take windows snap off. The resizing windows gets old real fast, but the massive amount of room is great. The down side is that you will find going back to single screen (or even dual) a chore.
