Allocate 2 CPUs and at least 2GB RAM to the VM. Make sure Windows has an adequate page file; I recommend 2GB minimum.
If you can, set your MacBook to boot using the 64-bit kernel and extensions. By default, most current Macs still boot with the 32-bit kernel. Google finds much info about this, including this page:http://newchaos.posterous.com/64-bit-on-snow-leopard-macos-x-106x
Combined with romant's prior advice, I saw a noticeable improvement, especially in terms of reduced typing lag in Visual Studio 2008. We'll see if this better performance continues tomorrow at work...