Don't fall into the "Stages" trap. More is not necessarily better. Better cartridges in the "stages" is what it is all about. E bay vendors came up with the stages game to promote their products, it makes them sound impressive until you look at what the stages contain. Things like granular activated carbon, very high or un rated micron size prefilters and carbon blocks, cloned unnamed membranes or 100GPD Dow Filmtecs not even intended for drinking water use or quality, small 6 or 9 oz horizontal chambers called DI filters, post RO taste and odor carbon filters etc.
Pay attention to what each stage contains and you are much better off. A very very good RO only needs 3 stages, RO/DI only 4. If you want to improve a system add a second full size 20 oz, 10" vertical DI like J_G did and you have 5 stages but they would all be beneficial not just a wide spot in the pipe as some are.
Rarely if ever do you need multiple carbons, you should never need multiple prefilters, a single vertical DI is many times more effective than multiple horizontals etc. Buy quality and use quality replacements and you should have a water quality tool that will last years and years with minimal maintenance on your part.
