My other PC is a Celeron 1.2 GHz with 512MB of RAM, and I can reach 12x at least (and I think that may be caused by using a 40 wire IDE cable). So I doubt it's the specs of your system that is causing the speed thing.
Memorex isn't the best media anyway - quality can be variable with it. Have you tried another brand of media?
What IDE cable do you have, 40 or 80 wire? Is 8x the maximum speed you are offered, or just the maximum speed you reach?
Also, checking your DMA may help, if only just to rule it out.
Just a few things to look at, don't know if it'll help.