A full system for $800? Sure, if you go to Aldi or walmart.com
Your money will go waaaay farther if you build your own.
Case, 30 bucks. don't waste your money on a poser aluminum one. Add a not-store-brand supply for another $30, and a fan or two for $7 each.
You should be able to find a decent motherboard for around $80, with on-board sound (you'll disable it) and ethernet.
Usefully decent processor, $130-180.
Heatsink w/ fan (if you don't get a retail chip), $15
512mb DDR memory, $40
A matched pair of IDE disks, about 80 each.
Promise FastTrak RAID controller, $90
Generic GeForce 4 video card, 64mb DDR, $120 or less.
Creative SoundBlaster Live, now down to around $40
Floppy drive, $20
CD-ROM, $30
That comes to around $800...All else you need to add is an OS (probably recycled from the machine you're replacing), and a pizza and a six-pack of dew to give your friendly neighborhood geek to put it together for you.
If Aldi still has their Medion PC available, it's $699, has a 2.66GHz P4 and pre-loaded WinXP Pro, and it's accessorized nicely as a media workstation. Just add a faster video card...