The best way to find the FAH 3d speed sweet spot is to do a little offline testing. Don't use live work units.
Obviously, you'll need the GPU client and the ATI Tool. Stop the client just after it writes a checkpoint or completed frame. Make a copy of the entire fah folder, and work with the copy. Run the -configonly, and change the setting for "Ask to connect" to yes (or just unplug your network drop while testing). This prevents the eventual EUE'd work unit from uploading. And then you can try it again.
First, you will want to find out what the default 3D clock speeds and voltages are for your card model. Also find out if the card defaults to 2D speeds, or always runs at 3D speeds. Google it.
Once you have that info, create a new profile in ATI tool. The first thing you want to do is increase the GPU voltage to 3D speeds and save the profile. And to save time, bump the GPU and Memory clocks. Half way up from 2D to 3D speeds. I also bumped all of the fan speeds by 5-7% for each of the temperature intervals. Not sure if that helps, but couldn't hurt, and doesn't change the noise level that much.
Okay, start the client. Watch the temps in ATI tool. When it reaches it's highest level, let it run for an hour or so. If no EUEs, stop the client, and bump up the GPU and Memory clocks 10% towards the 3D speed. Run the client again.
Keep going until you either EUE, or you reach 3D speeds (you can push it farther if you don't mind the fan noise.
). If you EUE, just make another copy of the orignal Fah client folder, back off the clock speeds a bit, and repeat. With a few hours testing, you'll find the sweet spot.
On my X1900XTX, it was about 15% over the default 3D speeds. I could go 20% if I cranked up the fan speed, but the noise and electricity usage didn't warrant the points increase. I could have gotten even more if I wanted to over-volt the GPU, but why risk a $300 GPU (price when new). CPU processors have thermal protection, GPU processors don't.
Sub 6 minute time frames are possible for top end cards, for over 900 PPD (as good as the PS3 client), but most people sit around the 7-8 minute mark for around 600 to 700 PPD. Hey, anytime I can drop a $150 (price now) upgrade in an old P4 system, and bump my PPD from 100 to 700 with little hassle or disruption, I'll take it.
Sure, for not much more, you could build a bare bones C2D and earn 2x the points. That's lots of fun to build a new system, but it takes time, and a lot of work too. You'll have to make that trade-off judgement for yourself.