A friend of mine uses his Gigasampler with a Celeron 500 (without overclocking), 320 MB (100 MHz) RAM, ATA66 hard disk (13.7 MB Fujitsu Picobird (7200 rpm)), AWE 64 Gold and M-Audio Delta DIO sound cards.
He seems to have no problems with polyphony when using Gigasampler on its own (with the Gigapiano). However, when using Cubase VST 3.7r2 at the same time on the same machine, he often experiences glitches on the VST audio tracks when playing the Gigapiano via midi. The glitches seem to occur at the time of hard disk accesses. He has tried many of the performance tweaks usually associated with Gigasampler and hard disk audio performance, but not much success as yet. He is planning to get a second disk drive just for the VST audio files.
However, a successful workaround has been to dramatically increase the VCACHE settings in the system.ini file (since he has lots of RAM). Having played back the audio and midi tracks once with all the associated disk accesses, the next time he plays the same sequence the hard disk light does not come on at all and there are no glitches!
You may want to wait two or three months to uprade or buy new computer gear. See my post about RAID controllers. Many of the overclocker sites are advising a short wait or you may have buyers remorse because of the new CPU chips and motherboards coming out soon.