You guys should be kidding... really made me register with this forum to correct misinformation.
Logic 8 creates threads for each channel regardless of the number and types (effects, insts, 3rd party/logic plugins) of plugins inserted, and yes it can use 8 cores.
But the current version of Logic seems to have problem with the new nehalem mac pro. Hyperthreading creates 16 virtual cores from the 8 physical cores and Logic's cpu meter shows 16 cores but currently it can use only 8 virtual cores. So you need to wait for the next update of Logic and Leopard.
L8 does use 8 cores on my 08 Mac Pro.
If you get a Nehalem MP, you'll need to wait.
And even after the updates, it won't be faster than the previous model until Snow Leopard and 64-bit Logic are out. It will take even more time until they become stable and 3rd party plugins become 64bit-compatible. And I'm pretty sure, at that time, there will already be even newer Westmere (Gulftown Xeon) 12-core Mac Pros.
So get the previous model and save some money.
My 2 yen