Already been asked a bunch of times and the answer is always no, you can't. The replays you are able to save to the PS3 desktop are merely instructions for the deck and game to render the video/audio. It's not a video like a .mov, .mp4, .avi would be so if you copy it to a mem stick, without the game and PS3 deck to render the instruction set, you get nothing. I've tried it too.
One way is to spool up the replay on your TV, then have a pc cam setup and aiming at the tv screen, maybe on a tripod for stability, then film it while it replays.
There's other ways, but this is the one I know of to get a race replay to your pc.
Another that I found on this site(imagine that!) is to route the audio/video cables from the ps3 through a vcr or dvd recorder then to the tv. Then just record the replay as it plays back. With the vcr you'd have to have a tv tuner card or similar in your pc to grab the stream from the vcr, but with a dvd just pop it in and rip it to the pc's harddrive. I'm guessing this is how some of the HD quality video's are being obtained.
