Does anything happen when you boot it? E.g Do the fans spin (cpu, case, power supply)?
If yes then unplug from the power outlet and reseat the vga, the memory modules and the vga EZ selector card. You see there is a possibility that something isn't making proper touch, especially this damn selector card. Then clear the cmos with the process described in the bios, reconnect with the power outlet and boot it.
Note: The memory modules should be in the A1 and B1 slots!
Now if you still have problems try booting with only one memory module. This must be in the B1 slot alone. If it boots go into the bios and set proper voltages and timings for your ram.