If I were you I would look on craigslist, eBay or even just asking my friends for some older gen pc hardware, like LGA 775 or LGA 1366. Then look for a power supply to power your GPU. If you realy wanted prebuilt, look for used business computers like a dell optiplex. In most case used business PCs will have decent CPUs but trash RAM and HDDs, so get one with a decent CPU then upgrade the RAM and HDD. *IMPORTANT* make sure the prebuilt is a full height case or else your GPU will not fit.