Upgrading whole system?

HI, my system specs atm are:

Dell Dimension C521
AMD Athlon 64 X2 3800+ 2.0gHz
Socket 940 AM2
2.5GB RAM DDR2
Nvidia Gforce 6150LE Integrated
PSU: 320W
Standard cooler


More here:
http://www.dealtime.com/xPF-Dell-Dell-Dimension-C521-Desktop-Computer-Athlon-64-3200-2-00GHz-160GB-512MB

Available slots:

PCI Express x16
PCI
PCI Express x1

And I looked around for new components to make this:

AMD Athlon 64 X2 6000+ 3.0gHz -> http://www.4allclients.nl/?action=4&id=675199#2
2.5GB RAM DDR2
Nvidia 8800GT -> http://www.newegg.com/Product/Product.aspx?Item=N82E16814143118

OR

Nvidia 8800GS -> http://www.newegg.com/Product/Product.aspx?Item=N82E16814150276

Not sure on what PSU or Cooling (if needed)

Then some questions:

1. Is it needed to buy a new processor? I am not going to overlock it!

2. How does the installing of a new processor go?

3. Is the 8800GT compatible with my system? will it fit?

4. Do i need a new cooler or PSU? Will that fit too?

Can somebody help?[/b]