If you are interested in building your own box then I suggest taking a look at this article from Ars Technica.

It is a great guide to building a low cost, but very capable machine.

I would suggest getting the base 20” iMac model and dual booting it. The price is a little over what you want, but worth it in my opinion. This gives you the best of both worlds. Here are the basic specs from apple.com:
* 2.0GHz Intel Core 2 Duo
* 1GB memory
* 250GB hard drive1
* 8x double-layer SuperDrive
* ATI Radeon HD 2400 XT with 128MB memory

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$1,199.00

Or, as lewis suggests, build it yourself. This can be a lot of fun and teach you a lot about how computers work. A great place for purchasing equipment is www.newegg.com. There are lots of customer reviews to help you pick worthwhile hardware.