You need to go with a ATT or one of the virtual networks which use their network. The ATT variant of the Nexus S doesn't support TMobile's 3g/"4g" frequencies.
In my opinion, the best deal is the Straight Talk SIM from Walmart which is $45 for unlimited everything:
Straight Talk SIM Card + iPhone 4S = $45 Unlimited Prepaid Plan My Money Blog
There are people that say there is a softcap on it. Pocketmobile also uses ATT's network. I'm not sure how cheap your roaming is on the Israeli providers, but I would think you'd come out cheaper with a virtual Israeli number forwarded to the US number and a cheap calling card back to Israel. I like VOIP Cheap (a Betamax company) with the Mobile VOIP app. 300 minutes a week are free to landlines, it supports VOIP over 3g, Callback, and dial through. 10 euros gets you 90 days worth of coverage.