There is no 757-ER.
Basic 757-200s have range (3,900nms) that is mostly un-tapped by Delta, and most carriers that use it, frankly. Probably its longest run for DL, which I think is seasonal, is ATL-ANC. That run is only 85% of its range. JFK-LAX is just about 60% of its range. JFK-HNL or ATL-HNL, however, is out of range, for this plane.
757s can be given ETOPS certification, which allows it to fly for extended times over water. ETOPS can be given in varying degrees, basically how far, in minutes, is it from a usable a runway. This doesn't impact the distance ability of a plane, just where relative to water it is allowed to operate.
As to the range of the 757-200 (DL's 7-5 derivative), it's just shy of ATL-SNN or ATL-LIS, and it couldn't make the return with the winds if it did make the outbound!
From New York, however, the 757-200 could easily reach DUB, LHR/LGW, CDG or DUS. CO uses 757-200s for several off-the-run European routes from Newark. IcelandAir uses the 757 on JFK-KEF.
757-200s don't reach Pisa or Budapest from JFK or ATL, places where a plane smaller than the 767 would make some sense.
See
Boeing's Site for more specifics on the plane.