Kyle Keen has written up documentation on his Rtl_fm utility, which enhances RTL-SDR operation on older PCs.
Rtl_fm is a little utility I wrote for the RTL-SDR project. The program was made to fill a gap in software defined radio: all the computers weaker than a Pentium 4. Basically, an Atom processor processor does not have enough oomph to demodulate something as simple as narrow band FM using the standard tools. (Recently a high performance FM demodulator was released, Simple FM but it works only passably on newer Atoms.) So rtl_fm was written with one goal, efficiency, in mind.
Rtl_fm is one of the user-level packages which comes as part of the rtl-sdr codebase.
Visit KK Blog for the documentation.