Honestly, with prices as low as they are, just get a new System Unit.
If you're going to replace the motherboard, get ready to replace everything else. New CPU, RAM, PSU, GPU, the works. If you try to match all the components to a new motherboard, you'll have to get one that is dated so everything will sync up--probably one damn close to the performance you have now.
So, side with caution and common sense and get a new System Unit. Keep all the peripherals and monitor, or maybe opt for a new monitor. Knocking off the monitor can save a bit, though. If it's old, don't try and upgrade from the floor up.