I have about same specs and i have no problems whatsoever, all settings highest.
Intel Core 2 Quad Q6600 2.40GHz,1066MHz
Asus P5K
Asus EN8800 GTS 512MB
Kingston DChannel 2GB Kit
And i am playing all kind of maps, i played a 3 star system with about 50-60 planets and had zero problems last weekend against 7 normal AI. I will check the biggest maps soon.
Are you sure you installed your motherboard drivers? thats all i can think about now, maybe some anti virus.... Could be all kind of things. A nice measure is also checking how many processes you got running on your pc when you just start your pc, i love it when someone asks why his pc is damn slow when i see two pages of installed applications when i press Start, Programs