More Memory?

Don't confuse hard drive space with memory (RAM). If you're running low on hard drive space you have a couple of options: replace the drive with a bigger drive (this would require you to reinstall everything and migrate your files over) or add a secondary drive or use an external hard drive to store files while you clear unneeded files off your main drive.

De-fragmentation is about hard drive. The stats you get at the end are about your hard drive and not your RAM. If you feel you need more space, then you should add more hard drives and not RAM.