mrvile, glad to see your taking single cpu advice :
Unless you're doing video editing, graphics intensive application (and i don't mean games), things of a workstation nature you don't benefit from dual cpu's.
If you have a good lcd, great go with it

Personally i have a 3000+ nad haven't hit the maximum overclocking point. It's stock 2.0ghz(or 2.2, but 2.0 i think), and have gone as high as 2.5ghz no problems and still have lot of room to continue going up. Your system will need a quality setup, wires will need to be decluttered, your fans will need to be effectively placed, the case should have good cooling properties.
The athlon chipsets don't get easier or harder to overclock. Some chipsets they sell are pretty much at their highest speed possible, so in that sense i guess they are harder. But just buy within the generation, and do some research, the cpu forum is a good place to start, and there are a lot of overclockers forums.