OK, I'll be upfront first, I'm a monster hater.
OK now to answer your question. Picture quality is determined by both cable quality, and connection type, with more emphasis on the latter. Here's a list of connections types for xbox from worst to best, and I've tried all of them.
composite cables (what your xbox came with, red yellow white) - sucks
S-video (advanced AV pack) - big improvement over composite, also I used AR performance series (the cheap blues ones) s-video cable instead of the thin black one that came with the advanced av pack.
component cables (hi-def av pack, similiar to the monster cables your interested in, and actually a great deal considering I paid just as much for the hi-def av pack) - I'm using AR master series component video, and optical audio cables with this. The improvement is incredible, whan xbmc is set to 720p, my friends claim that from the couch it looks almost as good as a computer monitor (i sit maybe 6-8' away)
My TV is a sony 42we655, a 42inch rear projection LCD. 720p is its native resolution.
Some stuff I need to mention is that with the advanced av pack, if i used the s-video cable it came with I get artifacts sometimes, like waves, or ghosting, or rainbow, it was only noticeable under certain colors, but it was annoying. But the problem went away when I used a cheap Acoustic Research s-video cable. Also with this if you kink the optical cable, audio will lose sync, it will lag compared to the picture.
During college, my room mate had a PS2 with monster cables, and I had my xbox with the advanced av-pack hooked up to his TV. The 20" phillips tv had component video inputs, but the tv was not a hi def unit, so there was little quality difference that we could tell. But chaning from stock cables, to monster cables on the ps2 did make a noticeable difference. One thing though, for some reason the monster cables would not sit tightly, it would loosen from time to time, and the display will go monochrome because of it, but just pushing it a little fixed that problem.
Now the hi-def av pack had great image quality since the day I got it, especially once I enabled 720p. I changed the cables only because I'm a dealer, and had them, not becuase I had to for any issues. I'd argue that the new cables did improve image quality slightly, but I did use $100 component video cables, and even though they cost me almost nothing, I think the placebo affect may be some cause of improvement.
Now I really doubt monster cables will give you a significance difference in quality, when my $100 cables failed to do anything noticeable. But then again you will be bypassing the hi-def av package box, while I'm not, so maybe there is some improvement in there. But since its $20, just get the monster and be happy with the placebo affect.