I've managed to get it to work on a GeForce 4 MX based card (which should indicate the age of the system).
It depends on just how much of the eye-candy you want to have turned on. Cranking up "flame on close", or "snow on desktop", or... most of the large(r) full screen effects really
slowed the system down. On more recent hardware, it was pretty but died with the lack of multi-monitor support.
Essentially, if you can't get it to work well, turn it back off. I tried it on an Athlon64 X2 5000 with a GeForce 7600 and worked brilliantly .... on the one monitor
. As soon as I had my dual monitor setup, I'd lose one and X would soon-enough need a restart (CTRL+ALT+Backspace).
YMMV. Good luck!