Would I be able to build a better pc for say $550?
I think we need to perform a comparison again. The original post specs an i7 930 with a 6970. Compared to the celeron with a 7770 for $600 (the 7770 wasn't listed in the $494 so I've added ~$100), I'm leaning towards the the i7 930 even if it is used.
Now. Did you find this on Craigslist, or is this a friend that's selling their computer? If it's from Craigslist, I would ask the owner to perform Sisoftware's SANDRA
benchmarking tool in your presence. It will take a while (I think it's 30 minutes or so but it's been so long I can't remember for sure), but if there are major problems with the computer, it will crash during the testing. If it makes it through - while it isn't conclusive - it should give you some assurance that the build is pretty reliable.
Hope this helps.