If I were you, the first thing that I would do is suggest that your client get a different video person. DV is atrocious for color keying.
There's information here about acquiring footage for color keying:
On that page, there's also a link to a handy animation preset, KeyerForDV, which makes keying DV footage a little better.
Everything Todd said... DV is pretty evil for anything that requires keying. Of course you can improve results by using tools that reduce the block artifacts before keying like Magic Bullet Denoiser, but generally you will only be able to go so far... Ideally you would record uncompressed to a hardddisk recorder via SDI or HDMI...
I like the links you've already been shown, but felt like tossing in a few more.
This offers some useful info on color resolution and would help explain why DV is about the worst choice for keying.
This post has an excellent collection of a lot of people's knowledge on keying. Very good bunch of know-how.
Here's a thread over on CreativeCOW that discusses getting a good keylight key.
Here's a tutorial on getting a decent key from a terrible chroma background.
Here's a tutorial on easily creating junk mattes that are really tight which makes keying much easier - especially on a poorly lit screen.