Currently PiPLAY doesn't include a N64 Emulator. Unfortunately N64 emulation has for a long time been extremely sloooowww.
Last year, Shea posted a tutorial on how to install mupen64plus-rpi by ricrpi which achieved almost playable speeds. In the video, he plays Starfox 64. It worked and was almost playable.

The original Mupen64Plus-rpi is now deprecated. However, ricrpi later released a much better version called Mupen64Plus.

Most games are still unplayable, but some such as Super Mario 64, Mario Kart 64, Star Fox 64 and Ocarina of Time are apparently easily playable.

Here is a (quite) impressive video demonstrating Super Mario 64 on the Raspberry Pi:


Skip forward to around 0:30 it's completely playable there. But it appears that graphical glitches are a bit of a problem though. (When Mario enters the castle at 0:40 for example)

Mupen64Plus still in development (last Github commit was 10 days ago) and hopefully the compatibility will improve. Until then it (probably) won't be included in PiPLAY.

The Raspberry Pi in the video above is only overclocked to 900mhz. That's also without overvoltage. It does however have the GPU memory at 128.

I hope this answers a few questions :D

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Congrats on reading through this ;) Here's some trivia:

Originally dismissed as an Aprils Fools Day Joke, there actually is a full ZX Spectrum emulator in the Goldeneye 64 code (along with the entire back catalogue of Ultimate games, Jet Pack, Night Lore etc.). Although deactivated before final release it can be accessed via a patch using an Everdrive cartridge on the original hardware (USA and JAP releases only, it was removed from the EUR ROM).