About the N64 games looking filtered... I believe N64 games rendered at 640x480. That was effectively twice the resolution PS1 games ran at.
Now, back then I believe most people would use a cable line or AV hookup to get to the TV. AV was fairly blurry, and cable was even worse. I can't recall if there was an S-video connector out for it or not. Point is, those connection types would seem blurry especially on older TVs. While the PS1 used the same connector types, the resolution was smaller, and it would result in less blur because there was bigger pixels being displayed. If you have a PS2, try using an S-video or Component after having it hooked up as Cable or AV. You will see a dramatic difference.
The bottom line; with non HD/older televisions, the larger the resolution is, the more the image will appear to blur, because it is packing in more pixels into the same screen area. The N64, I believe, had a higher resolution than the PS1, so more blurring was likely to occur.
This could be a possible explanation for the PS1 version looking clearer. It may just be our TV technology has increased now, not so much the game.
(Side note: PS1 can technically do 640x480 but it would increase render time too much, so one used it outside of say art galleries, ala Capcom VS SNK Pro.)