With any application, the scalability curve's slope decreases as processors are added: going from two processors to four processors might increase performance by 50% and going from four processors to eight would only increase performance by another 50% (this would be a logarithmic curve).
Database systems in general (and SQL Server specifically) will actually have more linear curves (i.e. better scalability) than most other uses if they are supporting many concurrent users. SQL Server is actually optimized for the TPC benchmarks which reward this scalability. Older Intel processors typically have a flatter scalability curve (but always poorer net performance) because memory speed has not kept up with processor speed. Windows 2000 Server has a flatter scalability curve than does NT 4. Considering where you work, you might want to peruse some of the Redbooks, e.g.:Tuning IBM eServers xSeries Servers for Performance.
--Jonathan