How do I Choose the Right Power Supply?

Hello. A few days ago the fan in my power supply started making weird
noises. The "whirring" sound of the fan would fluctuate like the fan was
slowing down. I talked to a man at CompUSA and he said the power supply is
going out. It's not even a year old!

The power supply is 300 watts and I would just buy a new one but I have to
think about the futu Right now I have an AMD Athelon for XP processor
with a cooling fan, 1 80 GB hard drive, an internal graphics card with a
cooling fan, an internal sound card, a CD-RW and an old CD-ROM I never use.
In the future, though I'm going to get a DVD burner (which will replace the
CD-ROM), an external Sound Blaster Extigy card, a new case with fans I'll
add, myself, a second 40 GB hard drive and an internal video capture card.
With all this, do I need to get a new power supply with a higher wattage? If
so, what wattage?

Here are my two BIG questions:

**How do I go about selecting the right wattage of power supply for my
needs?**

** Are there other points I need to consider?**

Thank you for any information you can give.

Jeremy