that means that I can say goodbye to 20Gb
Nonsense. You have not lost any hard drive space.
It's just a difference in labeling. It's like you purchased 4 quarts of gasoline and then found out you actually only got 3.78 liters. You didn't spill any gasoline, you're just measuring it differently.
Same thing here. By the hard drive manufacturer's measure, you bought 320GB. That's 320,000,000,000 bytes. By Windows measure, you still have 320,000,000,000 bytes, but Windows labels this 298GB.