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.
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.