Not quite what you were looking for, but this:-
http://www.fruitcake.plus.com/Sincla...aInterface.htm
-Provides SCART RGB output from the standard Spectrum, adds a joystick interface and RS232 interface and even some higher-than-original colour resolution video modes as well. (The Spectrum's original colour display only allowed two colours per character square, a major limitation).
To use this, though, you'd still need a standard-definition RGB video monitor like the rebadged Philips CMxxxx ones commonly used with the Commodore Amiga and Atari ST, or a TV with RGB inputs available on the SCART socket.
People practically give working TVs away these days, especially CRT ones, but I can understand you wanting to try to use a PC monitor which you already have. Problem is, the majority of PC monitors operate at higher line / frame frequencies than the standard definition output from the Spectrum.