MicroATX doesn't just have less space to work with, it has less ports.
My PC has a MicroATX motherboard. There is only one IDE port. There are like four SATAs, which is more important, but it limits your ability to use older hardware from past PCs. I had two IDE DVD burners and two IDE Hard drives in my old PC, and my new pc can only accept 2 ide devices, so I'm kinda just screwed there.
You are also less likely to have extended onboard functionality, ie connections for front IEEE, raid controllers, ect. You can also only put in 2 sticks of RAM.
Now, that said, I don't regret choosing my motherboard. It was dirt cheap compared to an ATX. Also, I used a MicroATX case with a handle that I've taken with me several times. I also built my entire PC for under $300 at a time when my CPU was the second best dual core on the market.
But, there ARE sacrifices to using an mATX mb.
I'd also like to throw out there...if you tell us what is wrong with your current PC, you can probably fix it, until you build your new system.
"Slowly dying" is an faulty way of saying your computer is breaking. Something is breaking. HD, Cpu, MB, Ram. Figure out which and replace that component and your PC will run fine.
Most commonly, in my experience, the HD begins failing and throwing up random errors and people assume its something really bad with their PC, when really, its not that bad. The same thing can happen with bad RAM or a dying motherboard, of course.
Antec 300 Illusion
430w Antec Earthwatts
AMD Q4 640 Propus
2x4gb Patriot DDR3 1333
750gb Seagate 7200.11 Hard Drive