I haven't had any experience in working with LCD's and xbox's but from what I have read you need a certain mod chip that will allow you the use of an LCD. I do not know which ones work and which ones do not work.
However I did buy a 16x2 LCD for my PC. I got it off of eBay on the cheap. It has a USB board, but the 20x4 LCD's without the USB board were either the same price or cheaper.
The LCD's have 14 pins that connect to the UBS board and if I remember correctly those 14 pins will easily connect to whatever mod chip will accommodate them.
These are the people I bought mine from:Sure Electronics
and I'm pretty sure that this is what you're looking for:20 x 04 Character LCD
you can also get them fromHERE
Diveno also sells mod chips.
Here is some info:CLICK!
I also found a FULL tutorial on how to install an LCD but the scene will not let me link to it. PM me and I'll shoot you the URL.