1) Windows SharePoint Service(WSS) comes with Windows 2003 server,
what can you do with WSS alone? Can I create an intranet with WSS and
Visual Studio.Net without buying SPS?

I would be leery to recommend WSS for an Intranet, given that
Intranets usually have large audiences and static content. It really
comes down to your site requirements. WSS is a collaboration tool for
small groups of people.


2) What I can do with SPS but cannot do with WSS ? Essentially, what
do I get with buying SPS?

Go check out some resources on this topic, lots of things have been
written:
heathersolomon.com/blog/articles/148.aspx#SP
heathersolomon.com/blog/articles/148.aspx#SPS


3) If I only need to create a project wiki that all team members can
have a say on it, do I need to buy SPS?

No, collaboration is WSS's strength.


4) If I only need to enable my team members to create their only
weblogs on our intranet, do I need to buy SPS? Can I get by with WSS?
My team are pretty proficient on Visual Studio and .Net programming.

Yes you can get by with WSS. You can create a site collection and
each user can have their own WSS site.