For £800 bucks? What is the system for?
This is the system I'd get for roughly the same price if I were on a budget for a Gaming/Photo processing rig.
Links are to a UK site, but I'm sure you can find the US equivalent for either the same price or cheaper.
Radon 690a AMD Bulldozer FX-4 4100 3.60GHz @ 4.00GHz Overclocked Bundle
^ Option to upgrade to 8GB ram for £16 (roughly 26 dollars)
OcUK HD 6850 OC 1024MB GDDR5 PCI-Express Graphics Card
^ You may not be able to find that exact card, due to it being made by OCUK (unless you want to pay them shipping as I think they do ship to US), but a 6850 is a decent GPU. I have a MSI 6850 and it runs like a dream.
That'd run you up to about 600 bucks. You'll need to spend the rest on a case, internal hard drive, optical drive (DVD RW/Blu Ray Reader) and a power supply.
Optical drives are nice and cheap, you can pick up a 500w (all you'd need really for this rig) power supply for about 60-70 dollars, from a decent brand (I use Cooler master)
The case and HDD will be a bit pricier, so the 800 may be stretched to 900 or so. If the 800 budget isn't negotiable, you may have to re look at the bundle I listed.