help new build

My motherboard of 8 years finally fried and I need to build a new rig but don't know what's good anymore.
I already have PSU,video card case and HDD. Just need CPU motherboard and memory, and a recommendation on a new operating system besides xp. The PC will be mainly just for working some video editing possibly
Also will my nvidia 7800gt work in the the newer PCI expressed slots and will my older Santa drive work in the newer ones?