Nokia had always said N-Gage would expand beyond Nseries models, but this is the first time they've stated a specific model. This is great news because it takes N-Gage one step closer to being a truly mass market platform. The 5320's very low launch price means a lot more people will be able to buy an N-Gage-compatible device, and the one thing any gaming platform needs is as large a userbase as possible.
UPDATE: The 5320 DOES have two gaming buttons! Rafe over on All About Symbian has been taking some pictures of a real life 5320 today at Nokia's publicity event. You can see the full photo gallery over on AAS, but the picture that interests us the most is this one illustrating both of the gaming buttons. The top button is also one of the music playback keys, which might look awkward but is actually in effect a shoulder button, so it should be fairly comfortable to use.
UPDATE 2: Apparently the 5320 also has special grips at the top and bottom of the back to make it more comfortable to hold in horizontal mode. You can see one of the grips in the publicity photo below, the other grip is hidden in the shadow at the bottom.

The 5320's gaming buttons
Here are some official pictures of the entire phone:



