Should I add a sump?

Some of you may have read the changes going on with my tank(s). My girlfriend and I both finally settled on being able to live with a 29g (well, she settled more than I did, but you know how that goes). Anyhow, before I get it set up, I am sort of toying with adding in a sump. I would basically modify that back part (not sure what that back part is called) by essentially gutting it and turning it into a coast to coast overflow and drilling 3 holes in the back for a beananimal style overflow. Not exactly sure what I would do for a sump just yet... I am FAIRLY sure the very back panel is glass painted black (feel free to chime in here as I haven;t found much info. on this although I haven't read up much on it either).
So... good idea? Bad idea? Mostly neutral?
The biggest reason I am considering this is because I am sure I want a skimmer, but the ones that fit inside these compartments are pretty weak. I have a bakpak that I could add onto it, however I would have to do some modification to both the skimmer and the biocube hood. The mods to the hood would be essentially irreversible, so I don't want to jump into that and decide something else might have been better!
On a somewhat side-note but still related topic: what is the footprint of a 20g? Is it the same as the average 10g? If so, I could probably make a 20g sump work very well!

 

20 long- 30x12x12
20 high- 24x12x16
20 XH- 20x10x20

Found that. Is XH for extra high or extra tall? The 20" width would be perfect. Where would I find this?