That depends... What do you want to use the computer for? If you just want to surf the web, check email, and work in Microsoft Office, you can get a great desktop for $800. If you want a gaming PC, then your choices are a lot more limited, though you can get a decent gaming desktop for ~$800.
Let us know what types of uses you'll want the computer for, and we'll give you some more specific recommendations. :-)
UPDATE: For $802.74 (not including shipping or an operating system), you can build a nice desktop PC that'll play games and video very well. Parts list (all sourced from NewEgg.com) below:
Intel Core i3-2100 Sandy Bridge 3.1GHz LGA 1155 65W Dual-Core Desktop Processor BX80623I32100
Item #: N82E16819115078
Return Policy: CPU Replacement Only Return Policy
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$124.99
GIGABYTE GA-Z68P-DS3 LGA 1155 Intel Z68 HDMI SATA 6Gb/s ATX Intel Motherboard
Item #: N82E16813128520
Return Policy: Standard Return Policy
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$107.99
G.SKILL Ripjaws Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-8GBRL
Item #: N82E16820231314
Return Policy: Memory Standard Return Policy
$54.99
EVGA 01G-P3-1371-TR GeForce GTX 460 (Fermi) 1GB 256-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
Item #: N82E16814130565
Return Policy: VGA Standard Return Policy
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$159.99
HANNspree By Hanns-G HF225DPB Black 21.5" Full HD WideScreen LCD Monitor w/Speakers
Item #: N82E16824262011
Return Policy: Monitor Standard Return Policy
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$119.99
CORSAIR Builder Series CX600 V2 600W ATX12V v2.3 80 PLUS Certified Active PFC Power Supply
Item #: N82E16817139028
Return Policy: Standard Return Policy
$69.99
Western Digital Caviar Black WD7502AAEX 750GB 7200 RPM SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive
Item #: N82E16822136794
Return Policy: Standard Return Policy
$64.99
ASUS DRW-24B1ST/BLK/B/AS Black SATA 24X DVD Burner - Bulk - OEM
Item #: N82E16827135204
Return Policy: Standard Return Policy
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$20.99
COOLER MASTER Elite 430 RC-430-KWN1 Black Steel / Plastic Computer Case
Item #: N82E16811119227
Return Policy: Standard Return Policy
$49.99
COOLER MASTER R4-C2R-20AC-GP 120mm Case Fan (for the case's back panel)
Item #: N82E16835103061
Return Policy: Standard Return Policy
$8.85
LITE-ON SK-1688U/B Black USB Wired Standard Keyboard
Item #: N82E16823107128
Return Policy: Standard Return Policy
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$7.99
Logitech B100 Black 3 Buttons 1 x Wheel USB Wired Optical Mouse - OEM
Item #: N82E16826104370
Return Policy: Standard Return Policy
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$11.99
SUBTOTAL: $802.74
A copy of Windows 7 Home Premium 64-bit (OEM version) is about $99.99. I also didn't include speakers since (a) the monitor has built-in speakers, crappy as they probably are, and (b) you should hear them for yourself before buying.
Hope that helped!