Bought an HP Pavillion a6500f (not great but got it cheap in a back to school tax-free deal), to play any decent games I need to up the graphics card (going with a GeForce 8500GT, again cheap), but in order to do that I need to upgrade the 250 watt power supply. I bought a CoolMax CA-400 (400 watt), but after installing it, when I push the power button the system comes on for only a second or less then shuts off again...HELP!!!

When I unplugged the original power supply there were 4 cables conneted: one large one plugged into the montherboard, one four pin also plugged into the motherboard near the cpu, one into the dvd drive, and one into the harddrive. I made sure I made the same connections with the new psu; when I started having the problem described above with the new psu I took it out and put the old one back in and the computer booted up fine. I made sure they were both ATX psu's but is there come other reason that the new psu wouldn't work (motherboard compatibility, etc...)?

I did and it works fine with old power supply, but I need the new power supply to run the new graphics card; do you think there is something wrong with the new power supply or could there be some reason why the new power supply wouldn't work with my computer?