I know its possible to put too little power into a M/B but is it possible to put too much power into it?
Yes, although most PSU's have built in protections to prevent over/under voltage. The use of a UPS (Uninterupted Power Source) will further aid in protecting your PC and your PSU. The UPS will regulate the electricity as to prevent any damage to the machine, and will also act as a battery backup in the case of power failure.
I have a new spare PSU but that is rated at 400w and my old one is 300w. Will it be safe to use?
Your 400W psu will be just fine. The 400W rating is more of a max than an actual push of power into your board. However, if your current psu is fine, then why replace it? Hence; "If it isn't broke, then don't fix it!". With that said, if you do choose to replace it, you may notice a slight increase in speed in your machine, or you might not if the 300w one was more than enough.
The AT, ATA is for the type of hard drives that your mobo will support. The most common ones are (and some one please correct me if I'm wrong) Serial ATA and IDE.
Let us know if you have any more questions!
Hope this helps!
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