Nehalem is amazing from a hardware point of view, but you have to sit back and ask yourself if the current software will justify it as an upgrade and the answer is that it won't. Penyrn and Conroe still aren't used to their full capability. Nothing really needs 8 threads atm, the only thing that likes 4 threads is my ubuntu installation.
Personally I'll hang on until the shrink to 32nm of Nehalem to even remotely consider an upgrade....