Ummm... custom builds are actually quite expensive. Buying a pre-fabricated one would be much cheap, but you probably will not get anything high quality for 800$. I've paid 1100 Euro for mine back in September 2006 and it's still only a multimedia notebook.

If you're in for good and cheap hardware, go for Acer. Their casings aren't the best though. If you're in for solid but expensive builds, SONY and Toshiba are a good bet (talking of experience, SONY lives forever and I haven't had any problems with my Toshiba either). I'd be careful with Samsung.