Apple Logic Pro /LogicExpress Discussion

Hi,
I'm looking at the new Macs and wondering if the construction is 
completely controlled by them and if any and/or all variations are also 
distributed by them. By variation, I mean less powerful compared to 
more powerful. 

I've only had experince with PC's and have built many for myself and 
others through the years and the gathering of parts has been as easy as 
walking through a computer parts shop and deciding how powerful I need 
it to be. 
All reference to Mac's has been as completely assembled units and my 
curiousity leads me to ask how far into the future the hardware that's 
incorporated with the units will go. Also, is the hardware different 
from that of a PC. Ex.. motherboard, processor, etc..

Any info would be greatly appreciated.
Thanks,
Gary