If you won't be overclocking then get 2 Gigs Corsair ValueSelect DDR400 RAM
You would save some money if you bought 4 x 512MB, but they would consume more voltage, and the performance gets reduced if you have all 4 DIMM slots running RAM. Don't remember the reason for that, but I think it gets underclocked to DDR333 or something like that.
As for Dual Core; If you're a gamer, then don't bother. If you're a professional who does officework and stuff, then you should consider it.
Personally, I wouldn't get dual core for either. The time to get Dual Core will be somewhere around 2007, IMO.