scpasco

G x G x diagnostic lights indicates as you said, a memory issue.

Are there any 'Beeps'?

Please try the following, using the 'old' memory modules:

 Install only one module in slot 1 and see if the system boots and if the computer starts normally, reinstall an additional module.

Does not boot, move the module to slot 2 and again see if the system boots, no boot, try the module in slot 3, no boot, try installing in slot 4.

Still does not boot, then remove the first module and using the second one, repeat the sequence described above.

Still no boot, try installing any remaining modules, one at a time, using the above procedure.

Continue until you have identified any faulty module [or slot] or reinstalled all modules without error.

Still does not boot, buy a new compatible memory module and try that.

Then if the computer still does not start, it's possible that the motherboard has failed and needs to be replaced.

Regards to your power supply question, higher wattage PSU's, than the 300w unit, are supported by the 5150/E510.

5150/E510 systems do not use Dell proprietary power supplies, you can install most generic ATX power supplies with either a 24-pin or 20+4-pin main motherboard power connector.

As the 5150/E510 has an open back panel, a power supply with or without the on/off switch can be used and can be purchased from most local or online computer stores.

A power supply with two SATA power connectors is needed.

I prefer the Corsair and Antec power supplies.

Bev.

The Seasonic 80 Bronze 300-watt is a good candidate for a replacement, as it has the right-angle SATA power connectors for the hard drives that the OEM unit has. Note that the SATA power connectors for the hard drives should be right-angle, as there is very little space between the backs of the drives and the side of the case; this unit has two right-angled, as well as two non-angled (for the optical drive(s)).

I replaced a power supply in a 5150 last year, and if I were doing it again I'd choose the Seasonic. (I used a Cooler Master, which is noisier and required an adapter for the hard drive power connectors.)

No beeps.  Yes, I tried the memory shuffle, no good results.  Thanks for the info on PSUs.

Any ideas on ways to check whether Motherboard is dead other than replacing power supply?

Thanks for the recommendation, looks like a good value, but I'd like to go up from 300watts to maybe 400.

Thanks for the recommendation, looks like a good value, but I'd like to go up from 300watts to maybe 400.

I have one of these in my XPS 7100. Quiet and rock-stable, but nearly twice the price of the 300-watt. 

If you're not interested in gaming, it's not necessary.