I'm building a computer which will primarily be used for business (so pretty much just MS Office, web browser, accounting software, etc, but a fair amount of multitasking), and some photo editing in CS5 on the side. I'm wondering whether the A6-3650 or the i3-2120 would be better for this, or if both are overkill and maybe something like the A4-3400 would be a better idea.
I like Intel for editing, but I haven't tried the new amd cpus. Whatever you get, get a matched 8 gb kit of ram; most are around $45 and I recommend 1.5v in corsair, kingston, gskill, patriot, crucial, or samsung (hynix). And this ecs board will work fine, only negative is a lack of an hdmi port for blueray: http://www.tigerdirect.com/applications/SearchTools/ite....
Get the A6-3650.
It is cheaper, has better graphics and 4 cores, so it is a better multitasker. The i3 would be better only if you plan to game and in that case you have to get a graphics card.
Also in any case I also recommend a good 8(2x4) GB RAM kit.
The A6 still isn't as fast as the i3, even in photo editing, but it should be ok. It depends on what you are personally comparing it to, ie - what you are used to using. Its about as quick as an athlon x3 430 or a core 2 due E8600. The i3 is about as quick as a core2quad 9650.
Both have integrated video chips, the A6 GPU being better of the two.
The I3 would dominate in transcoding videos because the i3 has a hardware encoder built in. It doesnt sound like you encode videos so this wouldn't be important to you.
If you decide on the A6, be sure to get qualified 1866 ram. Its been posted many times that theres a noticable performance gain over 1600.