If you've already got a decent S939 motherboard on your hands it would be almost criminal not to try an Athlon 64 3000+ E3-stepping CPU. Sure, it's not hugely fast at default speeds, but I'd be amazed if 2.5GHz wasn't a given and 3GHz on the cards with better cooling. There's just something rather naughty but nice about 50% overclocks with near-default voltage.
There's much to like about a processor that hits a 50% overclock with almost default voltage and goes that bit higher with a touch more VCore, especially when it only costs £100 or so in the first place. Overclocked using decent components that included a PIB Athlon 64 FX-55 cooler, our off-the-shelf sample managed a very respectable 2.8GHz with utter stability, allowing it to outgun a default clocked FX-55 in all of our tests. There's no obvious reason why you, the reader, can't emulate the results posted here, and it adds a whole new dimension to the performance-per-pound equation.