I know this response is a bit late, but perhaps it will help others.
I run VMWare and Windows XP on my 15" MacBook Pro. I do not know what the practical minimums are, but it is just a file. There is no need to partition the hard drive.
And clarification on what Andris has written. VMWare and Parallels are not emulators. Running Windows on a PowerPC Mac did require an emulator which would translate the i86 instructions to PowerPC instructions, and as Andris points out this resulted in significant performance degradation (I know, having used VirtualPC). However, when using a Mac with an Intel chip no instruction translation is required, and Windows can run natively on the Mac hardware. There is a performance hit when running VMWare or Parallels, because you are running Windows in an application on OS X, but in practice I've found it to be negligible. Andris is correct the running Windows via Apple's Bootcamp is the fastest way to run Windows on Mac hardware, as you are only running Windows and not OS X when using Bootcamp. But I've found the benefits of running both Mac and Windows applications at the same time to be worth any performance hit.
As I mentioned I use VMWare and can easily recommend it. I choose VMWare because it can use both cores of my dual core processor (as of this writing Parallels can use only one core), is a full Cocoa app, and has a $20 mail in rebate
. Several of my coworkers use Parallels and are as equally pleased (I work in an IT shop and do photography for fun). Both seem to be well thought out and well implemented packages.
Good luck and have fun whichever you decide.