If you want a N64 Controller Adaptor the [i[Apadtoid [/i]is your best buy. I have one - I really like it - and I use it for SNES,GBA,N64,and even sometimes PSX emulation. It has full support for Rumble paks, Transfer paks, Memory Paks, Keymapping, Complete customization, has a decent Windows XP driver set, and is basically the same as having a real N64 controller port hooked up to your computer - just with more features. More information is available
here
Personally I think that it is worth the $18 more that it costs. I haven't tried that PSX/N64 adaptor, but I don't think it is of *especially* high quality - I have see a few people who are having major problems with it - but you are better off trusting first-hand experience from them

.