You can prove the majority of you components, by simply having a "Start Up" floppy disk.
Put the disk in your drive and turn the machine on. If you now boot to a DOS prompt, the motherboard, graphics card and memory (at least) are OK.
However, if the machine still fails to boot, you are looking at a more potentially serious problem.
If you don't have a boot disk, one can be downloaded click here
Take a look at the obvious, all cables, memory etc. firmly in place. More often than not, when changing cards, something else gets disturbed.