I originally had a GT 240 video card but i bought a ENGTS 250 from Asus. After i installed it, my pc wont power on, a few times it does but shuts down in a few seconds. But when i switch it back to my old card, it boots just fine....
The most probable cause would be that your PSU is not quite up to powering the new more powerful card.
Yes i have just checked and the 250 uses twice the power of the 240.
I'm guessing you have had to rig up a second PCIE cable for the card using a 2x Molex to single PCIE cable adapter ?
There may be different versions around but the 250's i have seen use 2 X 6 pin while the 240 does not use a PCIE power cable.
As i said i have checked it out and there is no way you could expect to run that card on a 300 Watt PSU even a good branded one.