I've heard about the "perma-sleep" issue discussed in the Motherboards Forum, might search in there about it... but if you want more RC5 output, you probably don't want to put the system in Sleep mode anyway, as it throttles back the CPU (am I right on this with the A7V, anyone know?)
For maximum cracking, the #1 thing is really easy: run the blank-screen screensaver instead of something fancy. You can kill off background processes like Microsoft Office's "findfast" and other things that show up when you hit Ctrl+Alt+Delete, but in my experience the client is very efficient even when it has to co-exist with other processes. Your choice of a Duron CPU is one of the best for RC5 (clock-for-clock, the best of all is the Apple G4, *gasp*).
For cooling, I can endorse that plan to put a 120mm blowhole fan on there. I have had excellent results with that exact arrangement, it will put lots of room-temperature air right where it counts. I cut my blowholes with a fine-toothed blade on a power jigsaw. Also, if your computer isn't already on the floor, putting it down there instead of on the desk will reduce the air temperature a couple of degrees (hot air rises) and reduce your system temperature accordingly. If your power supply fan isn't already blowing air out the back, reverse the fan so that it is (be extremely cautious not to touch any of the circuitry, there is potentially-lethal amperage stored in there). Hopefully your case has an auxiliary 80mm exhaust fan on the rear panel? That is a big help too. You can increase the airflow by cutting out the metal grillework where the fan is pushing air through.
Welcome to Team Anandtech, by the way, it's great to see a new face here! : )