I just made the transition from PC to Mac earlier this year. I needed a new PC badly and something small and easily transferable for college from class to class...I was going to go for just another PC, but my friends at #vbfans persuaded me to invest in a Mac.
I decided on the Macbook with an upgrade to 2gb ram, and it was an excellent choice, and I have no regrets.
Regarding running lots of applications at once, not a problem, I usually run a myriad of programs while I am on the computer: Firefox, Zend Studio, Itunes, Adium, Word...without any lag whatsoever.
The only thing that I need (especially while programming) is an external monitor, which I plan on purchasing next week. So if you are planning on getting a lappy, keep that in mind....
Now, I am not a big fan of OS wars. I think they are pointless. Just go with what you want and what is in your budget...
