Are you using a passive DP to VGA adapter? Ive read some posts that speculate that they should work but never found anyone that accually tryed it and reported back. Are you also using both DVI outputs for your other monitors?
First, I installed the latest version of catalyst & video drivers. Then, I did a simple plug&play set-up. Everything worked fine on start up. All three monitors were running at their maximum resolutions and Eyefinity was working at 5760x1080.
I haven't done any extensive testing, but in the few hours I worked with the set up last nite, I did not experience a single problem.
Props to TheGreatGrapeApe for explaining (in another thread here at Tom's forums) that these passive adaptors do work for Eyefinity for monitors with resolutions up to 1920x1080 (and 1920x1200 with reduced blanking). If you plan to run at higher resolution, then you do need one of the powered active adaptors (afaik).
Additionally, (I've read) there can be some complications with using a passive dp adaptor when you are running monitors with different max resolutions (iirc, the dp adaptor must be connected to the monitor with the highest native resolution). Of course, eyefinity can only be applied extending the native resolution of the lowest resolution monitor of a display group, which is another complication of using a mish-mash of different monitors with eyefinity.