Frankly, Macs lacking USB 2.0 ports have hit obsolescence anyways.
If webcam chat or capture is important to you, spend your money on a new Mac first.
There's an old saying: You can't fit an elephant in your back pocket.
And if you’ve got an i Phone 4, there’s a great reason to do just that: 3G Face Time support.
When Face Time was announced, we all wondered if AT&T would let Apple (further) destroy their network by allowing the data-heavy service to be used over 3G.
Or it's the fact that you simply CANNOT BLAST 30FPS of High-Definition 720p or 1080p HD image data over a slow DSL or Cable modem connection, Period. Your expectations about webcam frame-rate may not be realistic.
Other reasons: Your computer may not be fast enough. There's a reason why almost all video chat/conferencing programs send smaller frames of video at 1/8 or 1/4 screen size: It's all that current affordble consumer technology and home data bandwidth can handle.
Launch i Chat, open your Video Preview window to put the i Sight in-use. Simply Put: Your webcam may have a very very hi-res HD sensor.
You may have to launch, relaunch Photoboth, un-plug, re-plug your external Mac web cam to get Photobooth to recognize it, but it's doable. But it's the video capture or chat programs you use often can't take advantage of it.
It was no surprise when Face Time was limited to Wi-Fi, though still disappointing.
But now, you can jaunt over to this page with your newly jailbroken i Phone 4, download the My3G app, and enjoy video chat wherever you are. Of course, if you’re on a tiered AT&T data plan, watch out, because this is, as expected, very hard on data.
However, only Windows software has ever been written to support them. i Movie is only designed to work with a i Sight on an i Mac or Mac Book - or a Fire Wire DV camera source.