I don't think 3G is gonna get anywhere near the 11mbps that USB 1.0 provides, so that shouldn't make a difference at all.
If you can throw bluetooth on that laptop, you'll have a better chance of getting a tether setup. Might need 10.4 or later for that though, I'm not sure.
You'll just need to create a bluetooth DUN (dialup networking) connection, and run something like PdaNet on the iPhone... or maybe MyWi supports BT DUN, I don't know.