A popular solution to this is to pick up a barebones starting point and add whatever you need that you can find.
A quick glance at newegg turned up this Foxconn barebones kit:
http://www.newegg.com/Product/Product.aspx?Item=N82E16856119013
Includes a dual-core atom processor, onboard everything.
109$
You will still need to pick up a hard drive, a dvd drive, and some ram. Along with the standard kb.mouse.card reader (if needed).monitor
This single stick of 2gb is the cheapest I could find on newegg (i'm not trying too hard here just giving you some ideas):
http://www.newegg.com/Product/Product.aspx?Item=N82E16820208344
2gb is the most the barebones mobo can support, its ddr2 800 which is faster than the mobo supports, but the dd2 533 is a couple dollars more, and the faster memory is perfectly useable, it will just be going at 533 speeds.
40$
Depending on how cheapass you want to be, you could get away with using a flash drive for your hard drive. 20$ is about what you'd spend on a 16gb, otherwise you need a SATA drive.
Heres a 3.5 80gb 7200 sata seagate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148231
35$
Lightscribe CD/DvD burner, standard 30$ ish
Heres one:
http://www.newegg.com/Product/Product.aspx?Item=27-106-335
32$
So we're at 216$+shipping for the core cost.
Microsoft Optical Mouse can be had for 10$ or less.
random google search shows it for 11$ here:
http://www.bhphotovideo.com/c/product/459024-REG/Microsoft_69H_00001_Comfort_Optical_Mouse_1000.html
Another 10$ for a MS keyboard:
http://www.compbargains.com/_e/Keyboards_Keypads/sproduct/11046949/Microsoft_Wired_Keyboard_500.htm
So now we're at about 236$ + shipping.
Our last two items are the monitor and some speakers.
The true cheapass can combine these into a single purchase. This monitor from some company I've never heard of offers speakers built in at a low price. The resolution isn't the greatest, but it does support at least 720, and I don't know how well the cpu/onboard graphics would do at a higher resolution. No DVI, but guess what? The barebones doesn't have a DVI port, so no loss there.
http://www.newegg.com/Product/Product.aspx?Item=N82E16824254042
112$
Speakers are probably shit, so you could look into a 2 or 2.1 sound system for cheap. Probably add 20-30$ for this.
This brings our total to about 350$ for everything. 375 if you want some external speaks.
Now, you might be able to get a better deal buying a premade computer, but I never do that and don't know the route you would go. However, 350$ for a "decent" computer is pretty damn good. It'll play media up to 720p without issue. Obviously its good for email and photo storage and word processing.
Don't think you'll be able to upgrade this at all apart from the hard drive.
----------------------------------------------------
Final parts list:
Foxconn barebones kit with Atom 330 Dual Core - 109$ +12.83$ shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16856119013
Transcend 2gb Stick DDR2 800 Memory - 40$ + 5.99$ shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16820208344
Seagate HDD 3.5' 80gb 7200 8mb cache - 35.99$ + 6.29$ shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16822148231
LiteOn LightScribe everything Burner - 31.99$ + 1.99$ shipping
http://www.newegg.com/Product/Product.aspx?Item=27-106-335
Microsoft Optical Mouse - ~10$ + ? shipping
Search around, but heres a link
http://www.bhphotovideo.com/c/product/459024-REG/Microsoft_69H_00001_Comfort_Optical_Mouse_1000.html
Microsoft Keyboard - ~10$ + ? shipping
Search around, but heres a link
http://www.compbargains.com/_e/Keyboards_Keypads/sproduct/11046949/Microsoft_Wired_Keyboard_500.htm
Hanns·G Widescreen LCD Monitor (1366x768 native) with Speakers - 112.99$ + 8.99$ shipping
http://www.newegg.com/Product/Product.aspx?Item=N82E16824254042
Total Price:
~350$
Grand Total with Shipping:
~386$