The cause of 3 red rings and E74 errors are GPU degradation (like nVidia problems with GeForce 7-8 series). CPU and memory do die quite seldom on these game consoles. You won't find any really new unused GPU for the X360, don't bother and don't waste your money. If you are still up to it, just check any ("new", "tested" or "with balls") IC you bought from eBay with microscope.
The best you can do is to reball GPU. They usually stay 4-8 months after what i see by repairing over a couple of hundred systems.
This post has been edited by soryu: Mar 10 2011, 12:54 PM