What version of the game are you running ?
I think older versions like 1.01 had memory leaks. This could explain the fact that your system is exceeding 2Gb in terms of total memory needs.
If you're not already running the latest patch (1.20 I think) then get it here
There's also a known bug but it only happens when ATI video cards are running in Crossfire mode... which is not your case.
Quote from this webpage
KNOWN ISSUE - ATI CROSSFIRE CARDS
- Crossfire cards with 256 Megs of memory, running in CrossFire mode will run out of memory at very high resolution settings.
On very high resolutions, such as 1920x1440 with 6x antialiasing. In CrossFire mode the ATI drivers are more restrictive about where resources are located, and as a result run out of memory sooner than with a single board. This running out of memory causes the device context creation to fail. The result is that the game just sits in a loop with a black screen. If you experience a context creation failure change your resolution to a lower setting (e.g. 1024x768 or even lower).