I have a 40X Memorex. The maximum rip rate I've seen it get to is 10X.
Be wary of Memorex though. Last year I bought my original 40X Memorex in April. It worked for a week then quit, so I returned it for a new one. In September, just 5 short months later, the replacement started acting up. It started by not reading certain burned cd's, then progressed to all burned cd's, and finally, it wouldn't read any cd. I tried cleaning it, but that didn't help. Eventually I contacted Memorex customer service. Over the course of about a month I placed several calls. Finally, after trying all they're "solutions" 10 or 11 times, they broke down and had me ship it in for them to run tests to see whether or not it needed to be replaced. Another month down the road, I got a package from Memorex. It was my cdrom returned with a message saying they couldn't find anything wrong with it. When I put it back in my computer, the drive wouldn't even spin up. It just made a low grating sound. I called them, gave them a piece of my mind, and finally got some semblance of the service I had originally expected. They paid for shipping, and shipped a new drive back immediatly. It was just before Christmas of last year when I finally got the replacement that I had asked for 3 months earlier.
One final thing, somewhere between the first time I called them and the last time I called them, they stopped their toll free 800 number customer service, and went to a long distance number.