Yeah, that happened to me with oblivion when i went from ati 9200 to x1900gt, when i passed from x1900gt to geforce 9800gtx+ didnt notice much change except that i could run games with AA and AF.
i have kinda the same setup exept my quadcore is amd but dont remember the numbers, my graphic card says 9800gtx+ 1gb but when i check in dxdiag, it says the graphic card has 1907mb of memory o_o.
The only thing i have to change in my pc is the HDD, i only have 2 160gb HDDs and both getting full so yeah :0.