Using mobile phone in USA/Canada

Anyone got any advice about the above ? I had my phone unblocked recently by my service provider - o2. It is a Tri-Band phone.

Can I walk into a Wal Mart (for Example) and buy a sim card and basically put this into my phone and use it as a "Pay as you go" or.......???

It should work. I have never really seen sim cards on sale without a phone in the US but I 'm sure you can get them. There are 2 different networks in the US, CDMA and GSM. Your phone will only work on the GSM networks (Cingular, T-Mobile)
Virgin mobile have a virtual network, but I do not know if it is GSM. I suggest you check out the websites for the above companies, there is probably a FAQ page or a customer care e-mail that you can contact with your question.