Eclipse IDE is not super tough on processors, but I don't see anywhere that it specifically says anything about how well it is threaded.
Either way, you can safely just get a 3570k and not worry about it.
Also, make sure you get a large SSD. You will want your OS, programming environment, and your programs that you are developing to all be on there.
- Edit - Best would probably still be the 3960X, or a high end Xeon or so, but the most cost effective high end processor is probably 3570k-ish.