I built a 3-core 3.2 ghz AMD setup for the wife about 4 months ago, win7-64bit home, 8gb ddr3 ram, cost was < $500 including windows 7 and I already had a tower which was an antec900.
What you have to decide up front building your own is do you want graphics power for gaming, or is that not a big requirement for you? If you don't want it, then building your own under $800 is very doable and i'll give you some info below.
fwiw I have an antec1200 case which costs around $170. I bought it more than a year before i built my current setup, with the intent of eventually messing with dual graphics cards and enjoying some games and maybe messing with water cooling. I never got around to that, but i am happy having the huge case and it will accept just about any kind of upgrade or modification. I also have 3 antec drive bays that let me plug in, or hot swap, full size hard drives like they were floppy disks into the front of the case, this i need for work and moving data and cloning hard drives. My setup which is not at all fancy, you will never be able to buy from dell, hp, or any store unless it's a store that builds custom and you will pay $2k+ for stuff like that. So think about what you really want, thinking about it is free.
now what are you going to do with the new pc? more than likely you can get by with a system similar to the amd setup I recently did:
$80 ASUS M4A88T-M LE AM3 AMD 880G HDMI Micro ATX AMD Motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16813131673
$83 AMD Athlon II X3 455 Rana 3.3GHz Socket AM3 95W Triple-Core Desktop Processor ADX455WFGMBOX
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103911
$65 (now $45) G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1066 (PC3 8500) Desktop Memory Model F3-8500CL7D-8GBRL
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231308
$55 Antec BP550 Plus 550W Continuous Power ATX12V V2.2 80 PLUS Certified Modular Active PFC Power Supply
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371016
$140 win7 64-bit
total around $420,
add $25 for dvd burner,
add around $70 per sata hard drive 500gb or better, SSD still not worth it too expensive,
you would need a case so around $100-150 for that,
and another $30 or so for odds and ends like sata cables,
that's around $700 using integrated graphics,
look for motherboard that has at least one PCI-E x16 slot for a graphics card that you can get whenever. And if you plan on getting a mid-to-high end graphics card plan on buying a higher end power supply (800w around $150) to support it.
and don't look at any of the rebates on newegg, you most likely will never get it they are basically a scam. instead search for stuff that has free shipping, and i don't recommend combo deals because you really don't save any money with them.
I also assembled the antec900 with motherboard, psu, cpu, in about an hour. It is not at all hard so don't let that worry you. Hardest part is figuring how you want to route cables in the case to your liking if you want it neat.