i havent seen too many amd builds with asus boards.
evga and foxconn (the two really high end mobos), biostar, msi seem to be the norm. if you are not going to overclock, you can even pick up an abit kn9 nf4 ultra ( i had one of these boards before my evga 590 sli, and it is very stable and actually overclocks pretty well) for like 70 bucks.
if you like the 70.00 less, consider this:
Newegg.com - AMD Athlon 64 X2 5000+ Brisbane 2.6GHz 2 x 512KB L2 Cache Socket AM2 Processor - Retail
this is an x2 5000, and not only is it 20.00 cheaper, it is a 65nm brisbane and it will run much cooler than that 90nm windsor 5600. if you decide to do so later, you should be able to buy an aftermarket hsf and clock it as high as a 6000. although, it has half the cache, and that will make a difference in some applications like video compressing and editing. some think it makes a difference in gaming, i dont really.
if you want to go really cheap:
Newegg.com - AMD Athlon 64 X2 3600+ Brisbane 1.9GHz 2 x 512KB L2 Cache Socket AM2 Processor - OEM
the 3600 brisbane, jewel in the rough for amd.
59.00 bucks OEM (oem because you will need an aftermarket hsf to hit high speeds, or get the retail and see how high you can get on stock, probably pretty high since the voltage increase is minimal)...virtually garaunteed to hit 3ghz with a 550 or 590 motherboard.
so, not only will it clock to x2 6000 speeds, it costs 1/4 as much.
remember, half the cache though. it really depends on what you are going to be doing (and how much of it) wether that is important or not.