I have slightly worse specs.
AMD 5000+ X2 2.6Ghz---4GB of DDR2 800 RAM---8800GTS 640MB I am currently running the Windows 7 64-bit RC. Latest video drivers as well.
I run at 1280x1024 with everything on highest, including AA. I have found that the game DOES slow down a lot on large maps with several very large fleets (at least 3-4 full fleets) but interestingly enough, it doesn't slow down anymore during battles. What I mean is, battles should provide a heavier load on the computer, and yet they do nothing to slow it down further when these large fleets fight. Anything below this amount and the game runs perfectly smooth. Turning off the AA seems to help a little, but not too much. I have theorized that this may be a game engine limitation, it does not "stutter" or "freeze" it just seems like everything moves similar to molasses, as if the speed is turned down.
Note: this is singleplayer.
The fact that battles don't slow things down even more seems to point to a limitation of the game engine. I am only playing vanilla sins with the latest patch, so I cannot say how things are in entrenchment.
Edit: the OP states that he is using a 64-bit OS therefore it will be using all of his RAM.
Second edit: I just reread what was stated, that Sins only uses up to 2GB regardless. A reference for that would be great if that was true.