Whoa. That contradicts everything I have ever heard comparing the two operating systems. Win 7 always sucks up more of my resources when I run bootcamp than OS X does.
For OP, you will notice a pretty big difference going from a quad desktop to a 1.4 C2D. The desktop has approximately 4x the processing power (2x clock, 2x cores), so it will be a bit of a step back.
Take video for instance... until just a few months ago, Apple didn't even allow third-party low-level API access to h.264. Apple is so far behind the game on things like OpenGL, and their graphics drivers have been pathetic. Go read up on the comparison of those things, and you will see what I mean.
People want to bash Windows, but the truth is it's a more "open" system allowing hardware to be taken advantage of and fully utilized. Apple commonly closes off everything, and they underclock and throttle things. Apple is getting better with their recent driver updates to the entire graphics system, but they're years behind Windows. In addition, with Windows, each vendor can write the absolute best drivers for their specific hardware. Apple allows no such thing.
If I had a 1.4 GHz system with Intel IGP, I wouldn't want to run OS X. I would be happy all day long with Windows 7 though. Take an HD video for example, or take Flash, and you will quickly see that Windows uses far less resources for the same tasks. Before the recent Apple change of strategy allowing h.264 API low-level access, my MBA would use 4X the CPU power to run Flash in OS X versus Windows 7. Those examples can be seen all day long. Look at why gamers need a PC... Apple just doesn't maximize and take advantage of hardware, and it leaves its users wanting more and NEEDING much more power in OS X to achieve the same performance in Windows at much less power.
Don't believe the hype you read about Windows being crap and OS X is everything. Windows has had problems in the past, but honestly it was due to vendors using crap components with crap drivers using the same resources. When Windows has decent hardware, and well written drivers, it can do everything very well.