I agree with the points that Korgasm made regarding the historical perspective. Technology in music as in all areas has marched on in almost 10 years the SY-99 has been around. The proprietary RAM cards made expanding the SY-99 as a sample playback unit pretty expensive though I'd recommend you grab any of these if you come across them as it expands the possibilities. Also the SY-99 had a superior effects processor to the SY-77. Side by side the 99 sounds smoother playing back the same patch.

The AFM engine is what really makes the SY-99 and SY-77 great synths. You wouldn't get now what you paid for the SY-99 (as with any older technology) so I'd recommend you appreciate for what it is and keep it for good. A musical instrument always has value.