You will gain nothing at all. When you have HT enabled Windows treats it like two CPUs and shows usage for each according to how Windows is allocating threads to them and how many resources are allocated to those threads (not how the threads and resources are running on the CPU). You should also notice that the idle process always takes up the remaining CPU power from what's currently being used. It doesn't mean the OS is using up 100% of the CPU's power all the time, it's just where those resources are allocated. In short, how Windows uses the CPU(s) and how it reports usage doesn't directly correlate with how much of the CPU's power is being used.
As it stands, Dolphin absolutely adores the Netburst architecture.. so much that, in it, my CPU significantly outperforms an Athlon 64 X2 4400+ @ 2.7GHz.