The G3 machines came out around MacOS v8.5.1; the OS (and the apps) have matured a long way since then. Is it still possible to get the spinning beachball in the newest Mac OS X (Snow Leopard)? Sure. Is it anywhere near the level you see crashing or bugginess on Windows? No.

You're also talking about an experience that comes from both the hardware and software. It is one thing to line up a PC and a Mac with similar specs, but an accurate comparison needs to factor in overhead introduced by the OS -- the newest Mac OS X (with apps) can actually run decently on as little as 1GB. More RAM gets you better performance, but on the low end, it doesn't have the bulky overhead that the Windows OSes do. The Mac OS X isn't all about the surface features you see in the GUI -- there is a whole different configuration underneath that doesn't have the legacy baggage of the current MS OS products. Some of this is probably an advantage of them being smaller and less prevalent than MS, the market sales leader.

I, too, wish Apple would release a small desktop machine with better expandability inside than the iMacs (maybe two disks inside, PCI Express slots), but less bulk and heft than the Mac Pro. Sort of a Mac Mini on steroids.