My experience is the same as Calum and TechnoGeek.
I feel like there is another issue here since I have run FAH SMP v7.1.52 on an AMD Turion TL-60 / 2GHz dual core / 4gb RAM laptop running 24/7, set to 100% cpu usage / lowest possible core priority. I had no problem with FAH slowing down any other application. I could stream movies/ You Tube vids ect. with no problem. Yes the CPU fan runs on high pretty much continuously, the CPU temp runs near high limits, and the CPU heat sink needs to be blown out often, but I haven't seen a problem with FAH slowing down other programs with any significance.
Just today on this HP Compaq 6715b machine I installed FAH SMP v7.2.9 beta set to 100% CPU usage / 'slightly higher core priority', and at this moment I can stream several You Tube videos simultaneously, and play a video game, and post to this thread without any noticeable performance issue.
I'm not sure what the issue is with your machine, but I just hate to see you run a uni-processor client on that Athlon x2 machine when you can contribute so much more running an SMP client.
I'm no guru, just my thoughts,
Mike