It'll do some things faster, like defrag. the drive. But if you ever run something that really needs the drive (some programs just don't like ram), it'll cause a major slowdown. You'll know it when it happens; most likely you'll interpret it as a lockup, and hit the reset button.
A viable alternative is to grab a registry tweak. There's a setting that makes Nes98/Me use all available ram before going to the hard disk. That's what I do, anyways.....