AMD A4-3300 Llano @ 2.5Ghz w/Radeon HD6410 GPU
GIGABYTE GA-A55M-S2V FM1 AMD A55
G.SKILL Value Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10600)
$180.15 (including tax and shipping, shipping may be more or less depending on where you live).
My best friend *just* bought the A4 3400, that same RAM, and an ASUS mobo for his Dad. I put it together for him just a few days ago. Not gonna' lie, if you want to play video games (with maxed graphics in all games), or do a lot of photoshop or video editing... probably not the system for your folks. Then again, Intel has nothing in the same price range that could compete in those areas either thanks to AMD's Radeon GPU.
However, as you said your folks will just be checking e-mail, typing in Word, some spread sheets in excel... that system would last them years to come, and would likely be noticably faster than their old 939 system. I am very impressed by the A4 3400 my buddy bought for his dad- thing is freaking amazing for how little it cost.
Anyhow, for the best bang for your buck while spending as little as possible- can't go wrong with AMD's APUs.