Currently quads only see an advantage in video and sound encoding, most programs and almost no games support more than dual core and wide spread support is probably a year or two away. I went higher speed dual core because currently games benefit more from the higher clock speed than the extra cores. I've been following the phenoms extensively, people have been going nuts trying to over clock them and getting no where fast.
The X2 6400 at $160 is pretty good but you have to keep in mind: It doesn't come with a cpu cooler, and you need a high end cooler to run it (zalman 9700, thermalright 120 ultra class) if your going to drive it like I drive mine. So the cooler is going to run 50-70 bucks extra. If you get a good universal it's worth it because you can carry it on to future builds.
I have a whole thread on the x2 6400 down in AMD Motherboards, benches and motherboard pics included here: http://forums.legitreviews.com/about13093.htm
I can get 3.7 stable with my cooling, with the Thermalright I got 3.52 easily with good temps. I'd recommend a Black Box edition of which ever cpu you choose from AMD as the unlocked multiplier makes it easy to over clock. On my board, a value based AMD 770 chipset the Asus M3A that runs $99, people have been getting 3.1 to 3.2 with the X2 5000 black. The X2 5000 runs a hundred and comes with a stock cooler (replace the stock cooler for over clocking).
IMHO it's better to go higher speed dual AMD than lower clock quad right now, and for phenom unless they do some thing to significantly improve performance and clock speed, and work out the bugs I'll never buy one.