This seems like a unnecessarily complicated, difficult and expensive way of doing what I've already managed to get set up for £4.
I could make this and sort out the sound but the only reason I can see anyone doing it is if they only had an HDMI port. In which case there are already (expensive) gadgets you can buy to do this.
But seeing as monitors generally have either VGA or dvi it can be done for £4. I'd do that, personally.
HDMI won't make an SD picture sharper btw. If anything it'll cause the picture to drop for a few seconds every now and then when it goes over the digital cliff.
Or maybe I've got that wrong but the point is that n64 can already be connected to almost any TV or monitor very cheaply and HDMI will not give a better picture it'll be exactly the same.
Monitors with only HDMI are rare. If ur in that position I sympathise because the solutions are expensive but there can't be many people in that position.