Another option you can go with is the AMD APUs. They combine the CPU and the GPU into one and therefore you can save the gfx card money...this is the set up I would go if you went with AMD's APU line of CPUs.
CPU: AMD A6-3670K $105
http://www.newegg.com/Product/Product.aspx?Item=N82E16819106002
The CPU combine the gfx card and CPU onto one card and therefore saves some money on the graphics card. It's not the best for gaming but will play FO3 with decent quality. This is AMD's newer version where you can OC the CPU which can come at an advantage. There are 2 and 3 core versions that are cheaper if you wish to go with those which will perform great with FO3 too although they aren't Overclockable. There is also a free DiRT3 coupon with purchase which is good if you like racing games...
Graphics: APU $0
You don't need a graphics card because the CPU doubles as a gfx card and it's a pretty good one too.
MoBo: ASRock A55M-HVS FM1 $59
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157277
This is the cheapest motherboard that supports the APU, it looks like a good motherboard too, and it is from a reliable company. The motherboard is socket
FM1 NOT AM3/AM3+...so be careful if you choose another motherboard. It also has a PCIe 2.0 slot for a graphics card upgrade later if you wish.
PSU: Antec EarthWatts Green EA-430D Green 430W $54
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371034
This PSU is a lower wattage than the other build because there isn't a discrete gfx card. It can even handle another graphics card addon later because it has enough wattage. It is from a very reliable company and should suffice for your needs.
Case: NZXT Gamma $35
http://www.amazon.com/GAMMA-Classic-Interior-Chassis-Black/dp/B002UDK9U6
A great case that also happens to be cheap. This case was designed for a gaming computer which means that it has room for longer graphics cards...although you don't have to worry about that for now.
HDD: HITACHI HDS721050CLA362 (0F10381) 500GB $80
http://www.newegg.com/Product/Product.aspx?Item=N82E16822145299
Same drive as other builds...cheap...effective.
RAM: G.SKILL NS 4GB Desktop Memory Model F3-10600CL9D-4GBNS $22
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231394
4GB is more than enough, also cheap and effective =D.
Heatsink+Fan: STOCK $0
DVD-ROM: ASUS 24X DVD Burner $19
http://www.newegg.com/Product/Product.aspx?Item=N82E16827135204
For DVDs and OS, nothing more to say..
Total: $374
Cheaper than previous builds and still allows you to play FO3 with decent settings and such. It also will allow you to convert to the good side: computer gaming
.
Also...Win7 for $90:
http://www.supermediastore.com/product/u/microsoft-windows-7-home-premium-sp1-64-bit-oem?utm_source=googleBase&utm_medium=pcsite&utm_content=Operating%20Systems&utm_campaign=googleBase%20-%20SF%20-%20SF-001-0038
If you're a student, you get Win 7 for around $40 from Microsoft I believe.