The only real benefit to be had from any modification is in getting the noise your computer makes down to an acceptable level.
Unless you have a very high spec PC or have overclocked components then you are highly unlikely to have a problem with it overheating using standard components.
Most mods are either made for marginal increases in speed - only really seen in tests and not noticed in day-to-day use or to make the box look better. I tend to view people who mod PCs in the same way I view those who mod cars and then clog the roads on a Saturday night.
A fan/heatsink on the processor, the graphics card and then one case fan is going to be more than enough for most standard PCs.