Sorry, you did say that in your original post. I'm an idiot.
From what I can see from the Acer website for this series of computer, they come with anywhere from a 250 w to a 300 watt power supply. A brand-name PSU like the 450 watt you asked about should be plenty.
This seems to be the Acer website for what you have:
http://panam.acer.com/acerpanam/desktop/0000/Acer/VeritonM420/VeritonM420sp2.shtml
Here's a good review of the PSU:
http://www.hardwaresecrets.com/article/Antec-VP450-Power-Supply-Review/1487
Here's a pertinent quote from their review (the Antec does not have a power factor correction (PFC) circuit):
"The Antec VP450 passed our tests with flying colors. Efficiency was between 80.3% and 85.5% during our tests, which is outstanding for a budget power supply. The only reason this unit can't get the 80 Plus certification is that only power supplies with a PFC circuit can be certified, as the 80 Plus certification requires a power factor of at least 0.900 at full load. Voltage regulation was outstanding, with all voltages closer to their nominal values than required (three percent regulation), except for the -12 V output, which was still inside the allowed range."
It should be more than enough for your Acer. I doubt the 250w or 300w PSU that shipped with the Acer is better than the Antec, and the Antec is rated for a much higher wattage.
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