Start-up Problem

Recently upgraded my desktop with

Gigabyte GA-MA78G-DS3H motherboard
AMD Athlon 64X2 Dual Core 6000 3.0GHz
4X1G 667 RAM

I have retained a PCI Express graphics card ? nVidia GeForce 7300GT and disabled the onboard graphics. Running XP Pro SP3. Power supply is 500W.

Now, some times, the PC would not respond to power on.
What can I do to fix this problem?
Should I use the on-board graphics instead?