Well I just read a preliminary review on Tomshardwareguide the Nehelam 2.93Ghz ran between 11% and 29% faster clock for clock compared to a previous Generation Core 2 Conroe depending on the test, though the current Yorkfield is not mentioned.
Remember this is still an unfinished Nehelam and using synthetic benchmarks. As tomshardware did not get enough time with the test unit for more thorough benchmarks. The final product will likely show a slightly better increase when comparing Ghz to Ghz as Nehelem is polished for mass production. One of the biggest advances though is a more efficient lower latency memory controller, more similar to what AMD has.
Nehelam also has the potential for higher clock frequencies and more cores which is where the real performance boost comes from. However, if you need a Mac I wouldn't bother waiting. Intel has plans for faster processors and new architectures down the line and will continue to do so.
Thanks!