<p>I have just moved job and gone from VB 6 to VB.Net and found the learning jump fairly steep, have more a problem with the object / conceptual side of things .. but getting there now ... but as I was a assembler / C++ 10/15 years ago and was considering learning C++/C# .Net (XNA games library calls my name) but not sure if it would hinder my VB.NET learning .... or should I just get myself certified</p>

To me the biggest obstacle for .NET is learn what is available in the framework. Therefore, if you find it easier to code in C# it will mean you only struggle with one thing instead of two. Once you know the framework it's just syntax really as 95% of the stuff you can do with C# can be done with VB.
Also, C# will force you to write code in a more object orientated manner as you can't fall back to coding in a VB6 style.
'
yield return
' is an example of something that doesn't have an equivalent in VB9 (there are rumours this is being added for VBX though).