Yes, you can download a VHD here that will work for 30 days: http://www.microsoft.com/downloads/details.aspx?FamilyID=67f93dcb-ada8-4db5-a47b-df17e14b2c74&DisplayLang=en

You will need Virtual Server R2, 1GB of RAM (2GB is better) and 10GB hard drive space.

The first question is do you want to focus on Configuration or Development? The second is do you want to specialise in MOSS or WSS?

There are two configuration exams:
http://www.microsoft.com/learning/exams/70-631.mspx
http://www.microsoft.com/learning/exams/70-630.mspx

There are also development exams:
http://www.microsoft.com/learning/exams/70-542.mspx
http://www.microsoft.com/learning/exams/70-541.mspx

In both cases there is one for MOSS and one for WSS.

If you pass any of these, you become a MCTS: Microsoft Certified Technical Specialist. This is like an MCP, as it only needs one exam passed to get, but it is a well sought after qualification and pays well:

http://www.itjobswatch.co.uk/contracts/uk/mcts.do

These exams are done through an exam provider like Prometric: http://www.prometric.com

You pay about $140/€100/£70 to sit the exam. There is about 50 multiple choice questions and you have 2-3 hours to do it. The pass mark is something like 70-85% so you have to know your stuff!

The simple answer is yes for Office documents you would collaborate on, but no for everything else! There are other issues like Offline access, cost of training and storage and migration.

Here are some great links that address these questions:

http://www.lcbridge.nl/download/limitsurl.htm

http://joeloleson.spaces.live.com/blog/cns!B05AD15E2DE730DD!347.entry

http://spsdreamjob.spaces.live.com/blog/cns%21D0B14C6B969D7501%21131.entry

http://blogs.msdn.com/joelo/archive/2007/01/12/file-servers-and-sharepoint-doc-libraries-tough-questions.aspx

http://www.lcbridge.nl/download/limitsurl.htm

http://joeloleson.spaces.live.com/blog/cns!B05AD15E2DE730DD!347.entry

http://spsdreamjob.spaces.live.com/blog/cns%21D0B14C6B969D7501%21131.entry

http://blogs.msdn.com/joelo/archive/2007/01/12/file-servers-and-sharepoint-doc-libraries-tough-questions.aspx

http://www.lcbridge.nl/download/limitsurl.htm

http://joeloleson.spaces.live.com/blog/cns!B05AD15E2DE730DD!347.entry

http://spsdreamjob.spaces.live.com/blog/cns%21D0B14C6B969D7501%21131.entry

http://blogs.msdn.com/joelo/archive/2007/01/12/file-servers-and-sharepoint-doc-libraries-tough-questions.aspx

http://blogs.msdn.com/sharepoint/archive/2007/01/02/is-the-file-server-dead.aspx

Some great news on the SharePoint team blog, on foot of making SharePoint DoD compliant, SharePoint will now be more accessable for people with disabilites, specifically the visually impaired. I have a physical disability myself, I walk with crutches, and so take a personal interest in this. And there are a lot more like me out there. Very welcome news.

http://blogs.msdn.com/sharepoint/archive/2007/09/05/pre-announcing-the-accessibility-kit-for-sharepoint-aks.aspx

Here is some more background:

http://www.21apps.com/2007/03/sharepoint-accessibility-is-moss-2007_4974.html