The blinking amber power light usually indicates a defective power supply. What power supply did you use to replace the original with? Do you know if that power supply is good? You need at power supply that is rated at least 350 watts.
However, if the power supply was replaced with a known good supply and it still does not work, the other most likely problem is the motherboard. A new motherboard from Dell, as reported by another user is close to $300. If if will cost you that much for a motherboard, realistically it's time for a new PC. There is a 3rd party company that sells Dell motherboards for about $100, I don't have any info on the company other than I've seen references to it on the forum (and I have referenced it to others but never had any feedback). Click Me
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