Are the samples on the same drive as the OS and Giga? This is a major contributor. Also as Chad mentioned, the sequencer on the same drive can be a problem. Also 80% (I assume you mean 80% showing in the GS window) has been found to be high for some users. Some folks report 90% with no problems, others get pops at 60%. Probably about disk throughput issues.
The key is: ALWAYS keep your Giga samples on a separate disk from the OS/GS program, keep them on a 7200RPM or faster drive with plenty of throughput.
IMO the memory percentage displayed by GigaStudio is the percentage of memory that Gst is able to \"lock\" from the total Ram memory. Locked memory is, well, locked memory blocks that cannot be moved in memory or swapped out to disk. The remaining small percentage should be sufficient for Windows to performs its mandatory tasks.
Giga uses locked memory blocks as buffers for the individual sample wave files with the Gig files.
I have also heard that people tend to think that this percentage should not be too high. Well, I guess that is prolly largely dependent on your amount of Ram.
I am not sure if the numbers are actually translatable back to megabytes:
On my 768 Mb system I can load samples up to 98% AND then still open Logic from within Gst (!). This would mean Logic would only see some 15 Mb free memory.