I only have a 500Mhz (AMD), 6 year old 4MB (!) S3 video card, SB PCI 128, w/ 256MB RAM, Win2K and I have no problems playing the demos. Sure the CPU gets up there, but never over 100% (onmymind.rns reaches about 98%).
Maybe it is because you have a Celeron, they are known to have crappy FPU performance, and, if I’m not mistaken, Renoise does everything using floating point internally.
my secondary pc, which is an Athlon 1.4Ghz (KT133A, 640MB PC133, lofi via onboard sound), plays everything just as good as my P IV 2.7Ghz does, proportionally of course.
situations in which my P IV (i845E, 512MB PC333, Sblive! classic, KXdrivers) reaches ~15% CPU load, the AMD rig is at about ~20%, which is pretty much the way it should be i’d say.
what chipset and soundcard is your 1.5Ghz rig equipped with?