Down Sizing Homebuilt Gaming Rig Advice

My current gaming build is nearing the end of its life and I want to replace it with something smaller and quieter, either a M-ATX or M-ITX build. i.e. something I could have in the lounge without disturbing everyone's tv viewing.

I haven't had experience of putting together builds for either size before, though M-ATX doesn't look so bad as the size difference isn't as great. I wanted to ask how viable is a M-ITX build? I know the answer to this will vary allot depending on what case you choose but in general is it possible to put a decent system together will full size GPU and no toasting everything, is it affordable or do specialist components like low profile cooling etc rocket the price...?

Are there any M-ITX cases that will take a full size power supply or will I need to go external/micro?

Would I be better off going M-ATX?

Budget excluding, Monitor, GPU (have GTX 670 already), and possibly power supply is around £500-£600 - feel free to suggest any specific builds.