Just to give you an example - I have a lot of Applications installed: a lot. But they only take up 50.7 GB on my hard drive - not even 1/10 of my 512GB capacity. If you're looking at a Retina I would get at a minimum to 512GB flash drive and 16GB of Ram because, unlike non-Retina machines, the flash drive is not, as of this writing, upgradeable and the RAM is soldered onto the logic board.
Program space is not a problem - having gigabytes and gigabytes of photos, music, video, etc., is. And they can easily be placed on an external drive (that's what I do with my 1 terabyte - I use my 2 terabyte for Time Machine back-ups and have an ancient LaCie 250GB that I just use as a 'holding' area.
If you want to play games, Mac's aren't the best way to go. To get around this you can either use Boot Camp, from Apple, to create a bootable partition for Windows 7 or 8, or use a virtual machine such as Parallels Desktop or VMFusion to play your Windows games. Now I'm not saying that Mac's don't have games - there are a slew of them - just not as many as on the Windows side.
Good luck - hope that this was of some help - call back with any other questions,
Clinton