You can use the vodafone sim card if u buy a sim adapter to make the 3G iPhone work , i have had mine working on Vodafone for the last 3 months and no problems , i have jailbroken and can install apps easily and still using 2.2 .
I would buy a sim adapter from
www.any-networksim.com for £30 really good mate.
The difference between the 2G and the 3G from what i can gather as i am still pretty new to this is that the 3G version has an extra bit of hardware that needs re-programming yo enable the handset to be unlocked via software.
Let me know if you need any more info,
I work for Vodafone uk so should be able to sort most things out .
Hope this helps.