The N64 emulation on the PC is fairly good, but there are still graphics glitches and lock ups. The entire console uses obscure hardware, and not much information is available on the inner workings. There were some developer manuals that were leaked, but those only show a very high level function of the system.
Since it would be very difficult for one standalone application to emulate the N64 reliably, Zilmar devised a plugin specification that would allow an audio plugin to process audio lists, a video plugin to process display lists, and an RSP plugin to cover other features of the RSP. The core would be maintained in the .exe and the plugins would be .dll's respectfully.
Xbox cannot use .dll's natively, so Surreal requires the plugin to be rebuilt as a static library, then linked with the binary upon compilation. When we were splitting the plugin combinations up to cut memory costs of bundled plugins, we had an obscenely high .xbe count. I think we were over 50 at one point. The biggest drawback is the CPU. We would really need an ultra efficient dynarec to play GoldenEye and Perfect Dark at smiling speeds. The scanners in Perfect Dark would need Jabo's or Glide64. And Glide64 requires a Glide3 wrapper. There's a closed source DirectX9 wrapper and an open source OpenGL wrapper. So there are a couple brick walls...
Back to PC side...
1964 has some new overclocking settings where you can play GoldenEye and Perfect Dark at 60 fps. Mupen64/Plus have been getting lots of attention as of late with PPC based consoles, better dynarecs, and about every new reversal of the N64 gets implemented. Daedalus has been in the middle of a backport of all of the progress made on the PSP. Hacktarux is working on a cycle accurate emulator, "a BSNES approach to N64." It may not be playing games full speed for awhile, but it sure will make it easier for future HLE developments and it is a step in the right direction.