Sounds like the MB or the video card. But im not that good at troubleshooting hardware. If it were the hard drive it would at least display something and just fail to access it on startup.
If you dont need the computer immediatly i would suggest buying any parts you may need online. Its significantly cheaper than wal-mart and if you order from a good site you will get em pretty quick.
www.newegg.com is probably the best one out there.