Three questions:
1. Do you have any parts from another computer (hard drive, optical drive, etc) that you are willing to reuse?
2. Do you intend on using this for gaming, and if so what resolution is your monitor? (This will determine how shiny your graphics card will be.)
3. Are you, or is someone you know, currently in college? If so you can pick up a copy of Windows 7 really cheap. If not, set aside $100 for an OEM copy of Win7 Home. Alternately you could save the money and run the Windows 8 previews for now, or run one of the trial versions of Windows off Microsoft.com.
There's no reason to not buy Intel right now. That i3 2100 is at least as good as any 4-thread AMD processor on the market today, and current LGA1155 motherboards should take an Ivy Bridge CPU. Even the dual-core Pentiums hold their own on the low end. So if you get a decent motherboard now, you can always drop in an IB quad later on.
Here's some random stuff to give you a starting point:
Radeon 6850
http://www.newegg.com/Product/Product.a ... 6814125353
If you order before the 12'th, the i3 2120 is actually cheaper than the 2100 thanks to a promo code. (It's the same as the 2100, just clocked slightly faster.)
http://www.newegg.com/Product/Product.a ... 6819115077
Biostar TZ68A+ motherboard. It's got the z68 chipset so you can take full advantage of any processor you throw at it, it's a steal at $110, and has a bunch of good reviews on tech sites:
http://www.newegg.com/Product/Product.a ... 6813138319
e: According to Biostar, all their 1155 motherboards will support Ivy Bridge.
[Source]
Corsair TX650 power supply. I know it's kind of expensive at around $80, but I don't *Can'tSayThisOnTV* around when it comes to power supplies. The number of amps on the 12 volt rail(s) is more important than the wattage they write in big letters on the side.
http://www.newegg.com/Product/Product.a ... 6817139020
You'll need RAM too, but luckily DDR3 is wicked cheap as long as you don't get 4321243MHZ XTREME OVERCLOCKING GAMER RAM.
For the case, you're on your own.
Appearance is subject to taste (I prefer well-built and subdued like the Antec 300), just make sure it can accommodate your motherboard. The power supplies that come with some cases aren't to be trusted unless you're spending $80 or more for the set. Sure they'll
work, but they're generally very cheap power supplies that make a lot of electrical noise and usually put out far more amps on the 5v rail than the 12v rail(s) to artificially inflate their wattage.
Unfortunately prices for hard drives are still high because of the floods, but the good news is SSDs are hovering around $1/gig. Take your pick but don't buy refurbs unless you get three and make a RAID 5.
Definitely sign up for NewEgg's email newsletters if you haven't already. They usually have a couple good coupon codes in them.
