Depends on what your looking for. Your def not getting much of a graphics card at that price. I would suggest a dell inspiron 1420. It's got Core 2 duo (only 1.5ghz) 2 gb ram 160 gig HD at the base $800. Going to have to pay more if you want bluetooth, webcam, case, or a graphics card. The Ubuntu version starts at like $750 if your into that. I really haven't seen a better deal in that range at least with good battery life.
Remember Dell has coupons online that will help even more. Up to 30% off on certain systems.
brw is right, you won't get much for 800. Look for a Dual Core (either AMD or Intel) at least 1gb or RAM and a 5400rpm HDD. If you can, try and get a low end graphics card. Integrated won't get you very far in terms of games.
Generally that means that someone bought it and had a problem with it. It was then sent back to the manu to be fixed and re-sold. I don't think it's bad if it saves you money. My desktop came as a recert fro Dell and I've had no problems with it for 3 years now. Check out the Dell Outlet, you should be able to find some nice stuff in your price range.