1) Yes, a change to the motherboard will be substantial enough to require a reinstall of Windows. As you have an HP computer, you aren't likely to be able to reactivate with the same code.
2) As others have stated, there isn't much that this system can still be expanded to. The socket is dying, as AMD is pushing more for Bulldozer/Piledriver. The power supply is reaching its limitations. The fact that it currently is a moderate gaming machine will be its fate, until it slips into a low end gaming machine in the next year to two years.
The sensible option would not be to upgrade the system. Save all of the money you want to put into a new CPU/motherboard. Get something Intel (either a used i5 2500k on the cheap, or a newer Ivy Bridge processor that fits your budget), as gaming appears to be your only goal. Reuse the RAM, PSU, HDDs, ODDs, and GPU to rebuild something that can last you more than the next 12 months. Next year, spend money on a good PSU, and potentially a new GPU. If you do this you can pull an excellent mid-range gaming rig out, rather than forming a piece meal rig that doesn't have any particularly finite goals
You haven't seen anything until you've seen this.
Wow! I didn't know the blue screen of death could get a blue screen of death.