First thing I do with a new Mac right out of the box is load a few custom made images I've made at the exact screen resolution native to the monitor size in solid colors of Black, Grey, Yellow, White, and Red to check for stuck and/or dead pixels.
Next thing is to insert the OS restore disk that comes with the Mac, boot from it, and wipe the internal drive using Disk Utility. I would not use a 7 or 35 pass erase, a single will do, as the others will take hours or even days. Then once the drive is wiped of the OS, reinstall it (you are still booted from the restore disk which is the only way to wipe the drive). Customize the install to take out all the nonsense you don't need, which for me includes support for booting into umpteen languages that I don't speak, read or write as well as not including all the additional language fonts, and the printer drivers. You don't need plug and play support for every printer made, just download and install the most current one(s) you will need for your printer(s) after setup. This will trim quite a bit of fat from the OS. You can also opt out of Rosetta support and if you don't develop, X11 as well. You will shave 3-6 GB off the OS space.
After the OS is reinstalled, run software update to get to 10.6.8 which just came out yesterday and won't be installed. You will also most likely need to install either from the restore disk or a 2ndary restore disk the iLife suite to get iPhoto, iMovie, iDVD etc, on there and can choose if you want all the iDVD themes and Garageband loops included. It will tell you the space all these items take.
I suggest not using the migration assistant and manually copying over data that you need as well as installing the latest versions of your apps from scratch with your license codes. This is your opportunity to clean up all your data rather than bringing it over in whatever mess it was in on your prior system.