Do I need a new power supply w/ my new Vid Card?

[CaucasianAsian29]

I want to install a Radeon x1600pro vid card in my new comp(getting it tomorrow!!). I already bough this card off ebay for $120.

My question is the card recommends a 350 watt power supply. But, my comp has a 305watt supply.
Wondering

1.Do I really need to get a new one?
2.What happens if I don't get one?
3.Is it difficult to install it, I was planning on having Geek Squad install the card for $40.I'm really scared to do it myself as I have never opened up a comp before.