I would also like to add that a video card is not needed for a computer to boot. If there is a problem where the system wont even pass the BIOS screen and shuts down right after wards, remove the video card to see if it helps.
Tools for ANY one that builds their own computer.
http://www.alwayslowest.com/AL/index.cfm?fuseaction=shop.dspSpecs&part=2952576 - Power supply tester.
http://www.newegg.com/Product/Product.aspx?Item=N82E16819116039 - if you have an Intel 775 system
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103195- if you have an AM2 system
http://www.newegg.com/Product/Product.aspx?Item=N82E16820134044 - Memory that will work in ANY DDR2 motherboard
http://www.newegg.com/Product/Product.aspx?Item=N82E16814161011 - PCI (not PCI express) video card. Newegg often has some great prices on open-box PCI video cards as well. Check your local ma-n-pa computer store, I'd be willing to bet that they have a working PCI video card somewhere in the back room for about $10.
Buying all of these components will let you diagnose almost any problem on TONS of different computer systems.