Many problems are due to cables and hardware connections.
At this point you should remove your mobo from the computer case and rest it on a box or stack of books beside the case.
The Minimalist Boot:
Disconnect everything that isn't required to boot, including:
Hard Drives, floppy drives, cd/dvd drives
All USB devices including the mouse and keyboard
All add-in PCI cards.
You want to boot only with your cpu and heat sink, PS2 keyboard, video card and only 1 memory module. That's it.
Make sure that all cables are installed properly and fully seated, including the ATX12v 4- or 8-pin connector.
Your initial goal is to be able to get into the bios where you will immediately load OPTIMIZED bios settings and save settings and exit. If this is successful, power off your machine and install your floppy or cd/dvd drive so that you can run Memtest86+, version 2.11 on the single memory module for10 full passes or longer.
If the problems remain:
1) With your computer off, turn off the power to the psu by using the psu's on/off switch.
2) Unplug the external power cord from the psu.
3) Press and hold the computer case's on/off switch for 10 seconds.
4) Leave your computer in this state for 15 minutes or longer.
5) Hook everything back up and turn on your computer.
You might have to perform these steps several times.
You might need to try different components:
Can you borrow any of these components from a friend?
If necessary, you can try using an older pci video card.
Once your system is stable, you should apply the basic settings in this post and set your DRAM voltage to your memory's max rated speed. Save settings and exit. Perform steps 1 and 3 above, then install a second memory module in slot #3. Boot and run an extended Memtest86+ on both memory modules.
Do NOT try using all four memory modules until your system is fully stable with all your hardware connected. With 8GB of memory, you might have to run your memory at less than its full rated speed. You should NOT install your mobo into the computer case until your problems are resolved.
There are several additional troubleshooting steps you can take if there you're still having problems.
A good psu with 400+ watts should be fine for modest setups.