New Computer Build - What do you think?

[Muffin Man]

What benefits are there to AM3 CPU's?

i had already answered this in my post:

just so that if/when you decide to upgrade again to a straight am3/ddr3 build, you'll be able to reuse the cpu.

i guess that would really be the main reason. you don't HAVE to get one, but it's nice to have that option to be able to reuse the cpu if/when you upgrade your rig in the future. and like i also said, you should be able to find one for around the same price.

Also, if I were to get an AM3 CPU, would I have to buy a different MOBO?

no. many am2+ mobos support am3 cpus, the mobo you listed being on of these boards. if you don't believe me , take a look at that mobo's cpu support list. you will find that it supports am3 cpus like the x4 945 and x4 955.
ASUSTeK Computer Inc.
(just so you know, yes you can use some am3 cpus on some am2+ mobos, but not vice versa. you cannot use am2+ cpus in am3 mobos.)