I recently converted to Mac OS X. I'm a computer programmer by day, and only really use my computer at home for email, web, dealing with pictures, downloading stuff and listening to music. For games I've got a DS Lite.
It's been a refreshing change moving to OS X. Initially, it's quite a shock how things are done differently - things like the dock, the location of the menu, how to install applications etc, but you should get used to it very quickly as it's very intuitive. A few days after using it I felt fairly confident. My girlfriend who has never used a Mac before managed to pick it up and just use it, as for her uses it's exactly the same as windows (Firefox and Thunderbird).
I have the bottom spec MacBook, which I've fitted with 2GB of ram and a 100GB HDD, using the initial 60GB as an external drive in an enclosure. Doing these yourself is simple and much cheaper than getting Apple to do it, and in the hard drives case, you end up having a spare drive - perfect for bunging in an enclosure for backups etc.
One thing to budget for is a memory upgrade - OS X doesn't play well with 512mb, especially if you try and run something intensive like Lightroom.