Hi,
I am no expert, but recently got a new PC to handle AVCHD editing.
A number of sources seemed to suggest that multi-core (quad) processors are best for this type of application, but graphics card doesn't make a hug difference.
Over the last 10 years I have built all my PC's, so looked into building an Intel Core i7 system, but came to conclusion at the time that I couldn't build an i7 core machine cheaper than Dell could build it for me (this was a few months ago now, so things may have changed). Cannot quite believe I bought a Dell, but in fairness it is very good and brezzes through AVCHD editing, sometimes doing renders quicker than real time

, which is amazing, although my videos are generally short with a few titles and transisions etc - nothing complicated.
All the best
Sheks