If you DO decide to get StraightTalk, buy the SIM card pack at WalMart - it's about $15, but comes with several SIM cards (Micro SIM, Regular SIM, etc) These will work on any GSM phone. It took a little bit to set up the MMS - so I could send and receive texts - but after that, it's been great!
Your Wal-Mart had the SIM cards?! My Wal-Mart was SO unhelpful it's not even funny, I had a ROUGH transition. It's all well now that I'm over, but I ended up ordering a phone from their website with a SIM and they sent the wrong type of SIM so I had to wait a week to get the right one. They make it sound like you can use your AT&T phone that takes a SIM but between the airheads in the store and the foreign people on the phone I gave up on that completely. The service is good though and the price is right so I try not to grumble too much.
I think that all companies have to port (or transfer) your current number. Just don't cancel your current service or it will get screwed up. And if you want to change your number this is the perfect time.