The most interesting will be to get your 4.0 certification. But this means that you will have to do a lot of studying since you've worken with .net 2.0, but not with 3.5/4.0, which as you stated, contain a lot of new features and possibilities.
Now, the basis for .net 2.0 certification is the same as for 3.5 certification (So you've already achieved the 536-certification).
For a detailed certification path for .NET 3.5 (Visual Studio 2008):
http://www.microsoft.com/learning/en/us/certification/cert-vstudio.aspx#tab3
For a detailed certification path for .NET 4.0 (Visual Studio 2010):
http://www.microsoft.com/learning/en/us/certification/cert-vstudio.aspx#tab2
Dimitri C. - Please mark the replies as answers if they help! Thanks.