This is the checklist mentioned above:READ before posting about boot problems!
#2 is the most common mistake.
Faulty RAM is also a common cause of your type of symptoms.
That's why people keep asking about the 8-pin CPU power connector and if you've tried with one stick of RAM.
A computer powering on but not displaying video output is very common and the reason the sticky linked above was created. The chances are very good that if you actually perform every step in the checklist you'll find your problem.