If so there seem to be a couple of devices to do this. You can get things like John Sims RGB to VGA adapter that will take PAL/NTSC RGB and convert to interlaced 15kHz VGA timings that might be accepted by the monitor. (best check with John.
Or you can use something like a ProV scaler to de-interlace and scale to VGA or SVGA resolution.
However, if you have a US XBOX with the HiDef pack the way to get highest quality would be to transcode the HiDef component output to RGBHV. That way you are getting the native rate of the game rather than an upscaled lower res version.
Not sure who else does them but Key Digital do a YPrPb to RGBHV transcoder that will auto sync to 480P/720P/1080i. Will retail for around £200 when they get to UK mid December.
The vga adaptor that lik-sang sells is a simple line doubler that will often produce a lower quality image than you will get on your TV.
However all region Xbox's have the ability to output progressive scan, and because of the official HDTV standard of US Xbox games the progressive scna Isn't taken out for pal games (The code is left in).
So two companies make VGA Adaptors for the xbox that utilise the progressive component signal out of the xbox and change it to Progressive RGB, which a monitor can handle. This will give you Dreamcast VGA box quality on the monitor.
Arguably the best is Kgaspers VGA box (xboxvga.xemulaton.com) however he has extremely long lead times (longer since you will need a PAL VGA box. The best to get is the DIgital X box (www.digital-x.de - you will have to search their site for the english section ) they seem to operate faster because they have more than 1 person making them. Both are around £65 including delivery, which is far cheaper than the key digital and audio authority boxes that produce worse results.
The best solution for Xbox Vga is to mod the box and use the region changer to set it as an NTSC box, this will mean you get higher compatability with PAL games, however this brings problems of its own with the LIVE service.