it really depends on what you are multitasking and what you are multitasking with?
for example I have a old PII 400 w/ 512 it running my shoutcast server, with the shoutcast server up, a couple internet explorer windows grinds the thing to a halt.
but my main computer is 2+Ghz w/ a Gig of DDR, I can literally have PhotoShop, Illustrator, outlook, 10 browser windows open and the thing doesn't hardly blink. but if I then tried to play a game with a the crap running it could be bogged down indeed.
I guess mainly what I'm saying is mess around a bit, find what your pc can and can't take by playing about with it.
FWIW, my system:
OS: Windows XP Professional SP 1a or Mandrake Linux 10.0 Official
MB: Asus A7N8X-E Deluxe
Processor: AMD Athlon XP 3200
Memory: 1.024 Gb DDR2
HD: Seagate Barracuda 7200 RPM SATA
BIOS: Award 6.0
4 case fans
Antec SOHO Server Case