With 1 GB Single Core I'd use this:
Typically use <Total System Memory> minus <Video used by onbord graphics if applicable> minus <256 - 384 MB, usually 320 MB for WinXP>
eg: 1,024 - 0 - 320 = 704 MB (+/- 64 MB)
eg: 2,048 - 0 - 320 = 1,728 MB (+/- 64 MB)
If you run two cores, and want to max each one, then divide by two, and use +/-32 MB per instance, so 352 MB between each instance for 1 GB, or 864 MB for each instance of Prime95 if you have 2 GB.
You'll also want Run FTTs in place left off.