What would be better to use?
Should I use MSMQ?
AJAX?
Something other?
First of all, it's important to understand that MSMQ and AJAX aren't mutually exclusive. They solve very different problems.
AJAX would improve the user experience for people connecting to your online poker game via a web client.
But as you've already implied, the principal challenge in implementing a massive multi-player poker game is the messaging infrastructure which includes the message bus and the ways by which your clients will send and receive messages.
So yes, MSMQ would be a good choice for the message bus (although it's by no means the only option). But as I said, in addition to the actual message bus, you need to think deeply about the client-side code on each platform which will send and receive messages. For instance, will you poll for messages? What should you do if a client has difficulty receiving messages? Etc.
As you can tell, the implementation of a messaging infrastructure is a complicated topic. Specific implementation details will vary by project so it's important to understand appropriate design patterns and heuristics.