Sound card are really good on-board nowadays, unless you are a audiophil I dont suggest chaning it.
Two thing, Dual-Core are generally slower in games than their Single core counterparts ebcasue they run slower.
Amd is by far the best machine for a gaming rig.
Would you be willing to consider a AMD rig?
Asus P4P800DX, P4C firstname.lastname@example.org, 2X512 OCZ PC4000 3-4-4-8, MSI 6800Ultra stock, 2X30gig Raid0