Sound card shouldn't be a problem, the onboard can usually be disabled in BIOS (although I often like the sound of onboard audio, but that's just me). Video, on the other hand, might be a problem. I don't know about Dells, but some computers with onboard video don't even have an AGP slot. You could add a PCI video card, but it wouldn't be much worth it probably.
And the Intel Extreme 3D doesn't even come close to a Geforce4. I wouldn't use it for much beyond very basic gaming.