Well, not being compatible isnt really a huge concern, its more along the lines of problems with power supply wattage/quality, limits to video card size due to the case size, airflow, things of that nature, and by the time you get everything to aleviate those kind of issues, you'll have pretty much replaced everything anyways.
I'm assuming you want to use the system for gaming.
You could probably reuse your old hard drive, ram(assuming its ddr3 1333 at least), and possibly the cpu, but if it were me, I'd upgrade to at least an ivy bridge, or high end AMD cpu.
If you can give us an idea of your budget, and what exactly you want your system to do, we can help you choose the right parts.