Side fan is not a bad idea, it helps to cool GPU, RAM, components on m/b(chipsets, etc). 25CM fan s run at low rpm therefor do not make much noise, at low rpm's they do not give any noticable pressure and cannot possibly blow against smaller higher rpm fans which produce more pressure. Keep one 120mm fan in front for cooling hdd's and put 1 or better 2 fans at the back/top plus PSU fan blowing out. You will need to have negative pressure in the case if You do not count side fan. If You will have positive preesure already before side fan, it will do nothing running @400 rpm as it cannot produce enough pressure to overcome pressure already in case, it will just stir air in itself.
Now for the dust problem. Lift up the case from floor, the higher You get it the less dust will enter the case. Add filters to intake fans. You can make the Yourself from pantyhose, they do filtering quite well but are quite restrictive and clog up quite soon and need to be cleaned regulary. I got fed up with them so I tried something else. I put fine alluminium mesh like this one:
http://www.halfords.com/webapp/wcs [...] yId_165625 in front of fans and it works quite well. Less restrictive to airflow, easier to clean, probably not filtering as well but case still stays clean. Pet hair and fabric fibre stucks in the mesh and small dust particle, fatty deposits( from sweat ond kitchen if it connected to room) and tar(if You smoke in the room) sticks to that hair and fibers on the mesh. Part of the small dust particles enter the case but are blown out as well because since there are no fibre, fat,tar in the case to stick to.