Yeah, you'll be fine. Photoshop starts up and runs very smoothly on the MacBooks. I would also suggest adding more RAM. I have 4GB now, but when I had the stock 1GB I was still able to multitask with very little lag.
I really like
Coda. If you don't know about it already, check it out. It's a text editor, CSS editor, and FTP application that will keep track of changes and upload them automatically. It's pretty cool, and there's so much more to it too.
Also, like someone else mentioned, if you don't have access to a Windows computer, then you will most certainly need to install it on a virtual machine (Sun's
VirtualBox is free) or use Boot Camp to install Windows natively on your hard drive and dual boot. Both will require that you purchase a Windows license.
If you're making sites for the average computer use this is what they will be using, and as unfortunate as it is, you'll have to test in Internet Explorer. There are tutorials that will show you how to install multiple versions of IE so you can test IE6, IE7, and probably IE8 on the same Windows install. I'd also recommend installing Windows versions of Firefox and Safari. Even though they are also on the Mac, you might run into a few issues on different OSes (usually the way form elements are displayed).