Are you going to buy a pre-built one or build it yourself? If there is a Micro Center near where you live, that's the best possible deal for do-it-yourself.
There are two philosphies for upgrading computers in my opinion:
1) Buy Awesome and upgrade when no longer awesome/old. That's the rout your $800 computer fallows.
2) Buy a deal and upgrade when it stops getting the job done. That will run you $300-$400 and you can upgrade every couple years.
If you want to build one, we can recommend parts. If you want to buy one, tell us which of options 1 or 2 you want and we can make suggestions.
jomoreb builds computers (http://www.tomshardware.com/forum/101313-8-blazing-fast-custom-built-desktop). I don't know about his prices, so you can ask around here if his prices are good. They'll definitely beat OEM (HP, Dell, etc.) prices.