Could it be power supply

Hello, a mate brought his comp to me with no pic , i have tried all tests but to no avail ... i have now put a new mobo in ASUS P5PE-VM , it has 1gb ram {1 stick} .... is there a chance that the power supply is not working correctly ? having looked at the mobo manual it says it needs a minimum 300w to power it , the one in comp is min 250w max 300w ..... any advice from anyone greatly recieved ..... also is there a test i can do to make sure power supply is working ok ... many thanks in advance for anyone that can help