Hi ~ I'll be in Israel and wanting to get to Greece. Would love to see Istanbul along the way, since the flight IST-ATH is $150, and TLV-ATH is $250.. I can't find whether there are buses from Israel to Istanbul ~??? Am interested in the cheapest solutions, of course ~ Thank you! Oh, and this would be next January or so...

No buses!
The cheapest solution is to fly from TLV to IST.
I can see TLV-IST one-way flights for January with Air Baltic via Riga for $154.
It's a long flight(9 hours) and ut sounds slightly silly to fly north to Riga than again south to Istanbul, but this is by far the cheapets and since you wouldn't mind a few days on a bus I guess it's allright for you.
[http://matrix.itasoftware.com/cvg/dispatch]
From Istanbul you can continue to Athens by train.

No - no direct buses at least. Any buses would have to pass Syria and that is not possible due to Syrian-Israeli relations.
That said you can take a series of different buses changing at Amman, Damascus, Adana and Aleppo, however for this to work you would have to :
1) avoid getting stamped in and out of Israel
2) fly from Tel Aviv to Amman to avoid a Jordanian entry stamp from the Israeli border.

If you are in Israel go down to Ben Gurion airport and talk to the the people there.
I heard there are sometimes cheap charter flights from Tel Aviv to Istanbul, you might be able to find flights from Tel Aviv to Istanbul for 200 USD return. AFAIK the way to get these cheap charter flights is to buy the tickets at the airport a few days before departure. Talk to the travel agents working in Ben Gurion, they might know more about this.
You should also wait until late September to book your flights, because late September is the time the charter and budget airlines get their winter schedules online. Last year it was possible to fly to Germany from Tel Aviv for 150 USD round-trip with Tuifly and you can usually get very cheap flights to Tessaloniki, Istanbul and Athens from Germany.