How much memory do I need to run the program?
You can basically run the LTspice IV if you can operate your Windows system. We have spent a great deal of effort in minimizing the memory requirement of this program. While a typical simulation might generate 8Gigabytes of raw data, that data will be compressed on the disk to 400Megabytes. To view a single trace would require less than 65Megabytes of RAM. Of course, the more memory the better the performance will be. Also, LTspice IV benefits from the improved memory performance of Windows NT and later operating systems, so you might consider upgrading operating system if you run out of memory. An x64 OS will be the best choice in this regard.
Where is the waveform stored during simulation?
All the waveform data are stored on hard disk. Only the plotted traces are loaded into RAM. Turning off the marching waveforms can reduce the RAM memory requirement. Note that for most analysis types, there is no particular file size limit. You can generate and view .raw files that are very many Gigabytes in size.
What if I don't have enough disk space for long simulation?
The waveform data has been compressed, but it is still proportional to the run time and the number of traces saved. The easiest way to save memory is to select desired traces for storing before the simulation starts.
OK, I've done everything and I'm still running out of memory. What can I do?
During a transient analysis, you can interactively throw away the past waveforms by pressing the '0' key. That will retrigger the simulation time to t=0 as the present time.