I would make 100% sure that the GFX card is sat properly, I've built loads of machines but I still sometimes leave the AGP card sitting out and not quite connecting properly. Failing that do you have an old PCI gfx card sitting around? Sometimes MOBOS are set to boot from a PCI rather than an AGP card (it's a setting in your bios) I've had that solve a similar problem before (although it was a few years ago now). You can't put your ram i the wrong way round as there is a notch that prevents you from doing so, or do you mean in different slots? Check your manual for the best configuration for what you have. If you have only one stick of RAM put it in bank 0.

Huxleys suggestions are probably the best route for you to take now, check each piece methodically. And if you do have your machine running and then shutting down after 1 minute it's almost certainly overheating. If it does overheat you probably will have to clear the CMOS so that it defaults bake to safe settings (that has happened to me recently with my board). But I odnt think you've got enough paste on there at the moment, Make sure that you get a good connection between the components.

Good luck, I know how frustrating it can be

Alan