The NGage ist still gamingwise one of the best phones you can get since gaming was its primary design goal. No other mobile I know has a true 8-way joystick but the Ngage does. It has stereo sound speakers and is still one of Symbian phones with the most main memory.
Consider that all Symbian phones have a very low resoulution. 176x208 is not much and it takes a lot of scaling and clever blitters to get a good fullscreen picture on the NGage screen. Another downside is its 103 MHZ processor that is not enough for most systems to run fluently. I have a 6630 which is one of the few Symbian phones that have a 206 MHZ processor. Still even the 6630 has 2 MB less main RAM than the NGage. Also the NGage sound output like all Symbian devices is very bad. If you want to use your phone as an audio device (MP3's, videos) you need anewer generation of Symbian phones to get CD quality sound, e.g. The SX 1 Music Editon or the 6630 (that's why I have one).
Softwarewise you can get some nice fullspeed emus for NES, Genesis and Gameboy, some WIP SNES. All of these are commercial emus but they have very good speed at least on my phone.
EMama doesn't support the 6630 (yet, I hope) but the range of games that are playable is very limited. Mostly games from the Pacman and 1942 era. No CPS1 or Raiden I'm afraid. Speed on the few which run is supposed to be quite playable, as I've heard.
Also you should try to get the NGage QD, if it is just gaming for you. It's smaller, also hotswapping the memory card. You have to powerdown the original NGage to replace the memory card. Also smaller.
I'm a little suprised that companies are still giving out NGages. In Europe these phones were so unpopular that they were discontinued by Nokia even before the first batch was sold out. If you can get it for free it can be nice to play Tomb Raider on the road, but as for a serious mobile gaming device the NGage is to slow and the range of games is to lim ited to provide a real gaming experience. And I'm not only talking about the NGage games but emus too. As said the NGage is to underpowered to be a target for future Symbian emu programmers. The focus will shift to the faster 206 MHZ phones.