That being said, it may not be the best option, you have to consider price as well. So you could get something like the AMD Athlon 64 X2 5600 which costs about £50 less and has the 2 x 1MB L2 Cache. You could probably even overclock it to about 3Ghz maybe more. With the 6400 Black Edition i doubt you will get much more out of it, maybe 3.4Ghz but i haven't seen much about overclocking it yet, the only thing i did see was one where they got it to 3.57Ghz, but that was probably with some serious cooling.
On another note, are you planning on buying this motherboard, or do you already have it? If you are planning on getting it, then i suggest you look for something with an nForce 570 or better yet an nForce 590 chipset. That GigaByte board you are talking about uses the nForce 4 chipset which must be the earliest chipset used on AM2 boards because i have an nForce 4 chipset but it's a 939 board, the previous socket.