Hyperthreading is a total joke when it comes to rendering or any other computationally rigorous application. Unless you have an application that is perfectly optimized for HT, you will not see a performance gain, and in most cases you will see a performance decline as illustrated by Max3DD3. Best to turn it off in the bios as see for yourself. The reason your P4 shows up as 2 cpu's is that HT is turned on, which is a software gimmick that makes the OS think that there is 2 cpus, when in actuality there is only one.