It also depends on video card.
On my Cel900/V5 6000 some games is not too fast but others is about 200-500% w/o frame limit/vsync.
2xOpteron 244 w/ V5 5500PCI - no slowdowns at all but dual Opteron is overkill for N64 emulation. Athlon-1700/P4 2.0 is more than enough for all emulators except Dolphin and latest games in MAME (Seattle hardware - slow cpu interpreter plus (OMG !) software 3dfx Voodoo emulator, with real glide it goes 40% faster)
For N64 optimal machine is something like AthlonXP 2500/1Gb/Radeon 9700 + Voodoo 5 5500 PCI)
-1.2GHz will get you good frame rates in most games.
-2.0/2000+ GHz will get good frame rates in the high system requirement games
-Above that you'll get good frame rates in some of the "slow" games.
It's the processor that determines the frame rate most of the time, so an very cheap upgrade to a faster Celeron/Pentium that will go with your current motherboard would be great if you want to boost the frame rate. 1964 0.9.9 is also much faster so you should definitely try that out.