Sony's about the only brand that comes out "ready to video edit" that's probably worth getting. HP has some video editing statinos, but I don't really trust them.
As for memory, 400G is just plain over kill. I use 80 on my video computer and have done just fine. A 5.1 sound card would be a waste as well because firewire is simply 2 channel.
If you want a REAL video editing station, save up about $5000 and look into some DV Line system with a DV-Storm in it. The one my school had used duel 80G and a 40G to boot from. Duel processors are nice, but you pay for them, and I doubt you need the real time editing, as you get with those.