You CANNOT transfer video from your camera using USB. And yes, your G5 has USB - that is how your keyboard and mouse connect. There are two USB jacks on the back of your machine. Plug the keyboard in one, plug the mouse into the keyboard. Te only thing that USB connection to you camera can do is transferring stills from the camera - not video.
So you figured out that using the FireWire cable is how to transfer the High Definition video from the camera. Great! The video you are importing gets buffered and "processed" during the import phase. Your G5 does not have a FireWire 800 connection - and even if it did (if you add one), it will not increase the processing time - it is not the FireWire connection.
The ONLY way to speed up that import speed is to increase the CPU speed - that is, replace the computer with a newer one. For real time, high definition video transfer, a MacPro tower will do the trick.
If ALL your video import is happening at 1/4 time, I do have a couple of suggestions, but it will not get to real time:
1) Max out your RAM. Electronic - REAL - RAM will help.
2) In case you have not already noticed, high definition video will take up about 4x more space on you hard drive than standard definition video. You may need another hard drive. Since you did not tell us which G5, I am presuming a G5 flatpanel - without the camera. An external hard drive connecting using FireWire will work just fine.
I use a G5 flatpanel iMac (2GHz CPU, I think). It maxes out at 2 gig RAM. I use an external SIIG hard drive case with 250 gig drive (bigger is better). My applications stay on the internal drive; the video files stay on the external drive(s). I use a Sony HDR-HC1 and have been editing high-definition video using iMovie and FinalCut Pro for well over a year. Transfer of high definition video from my camera to my 'puter is between 1/4 to occasional 3/4 time. Using my son's CoreDuo Intel-chip based iMac flatpanel results in 1/2 to occasional real-time transfer.
In either case, standard definition video is transferred realtime on either machine (even on my older G4 tower).
I don't start the import and just sit there... rather, I know how much I shot, so I start the transfer and go do something else. I will occasionally check on it, otherwise, when the contents of the tape have been imported, the buffered video continues to be processed, the camera will keep going until the end of the tape then automatically stop when the end of the tape is reached... the video apps are smart enough to know that there is no video coming over, so you won't have any wasted hard drive space - if the camera is blue-screened (not playing any content), then nothing is being imported or buffered or taking up hard drive space.
I hope this helps.
I just got an fx7 and I'm new to HDV. I am using an iMac G5 (the older one without the built-in camera) and I tried importing my footage with the usb chord that came with the camera but it's not compatible my computer it only uses firewire. So then i tried my fire wire chord from my old camera and it worked but was importing at 1/4 speed. What do I need to get to import at regular time? I was wondering if an 800 firewire would work.

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