I'm playing on a machine I built in '06 and have the settings mostly at medium, with shadows off. It runs pretty well. Only really see lag issues when playing online which likely points to network bottlenecks not proccessing power.
C2D E6400, one of the earlier dual core chips
Gforce 7950gt
2gigs of ram at 800mhz bus
I built this rig when I heard about Supreme Commander launching in Feb 07 and knew my previous rig wouldn't cut it. If you can run SupCom, you can run DG as it runs on the same engine. I don't think the specs are at all unreasonable for a 2009 AAA title.