I'm reluctant to deal with individual verses before the larger issues (uniqueness of salvation, nature of God and sin etc) are resolved but this came up in the comments of the last post. I'm no Greek genius but the evidence seems very clear.
"And these will go away into eternal (αιωνιον) punishment (κολασιν) but the righteous into eternal (αιωνιον) life.”
According to BDAG, (the gold standard of this sort of stuff) κολασιν means retributive punishment and αιωνιον means a period of unending duration, without end. These meanings make sense of the wider context, chapter 25, where Jesus gives an extended description of the final judgement. Furthermore αιων, which I presume our English word for eon/age comes from, is a slightly different word and of course includes the meaning of a limited age (Mt 13:22) but can also mean "the Aeon person" (Eph 2:2) or a non-specific period of time (Lk 1:70). In addition in Matt 25:46, it'd be nearly impossible for αιωνιον the same word, used in conceptual parallel, to have opposite meanings!
[Updated post: Greek Font display fixed, thanks Marc, such an obvious solution! You should all regardless of browsers see the words in Koine Greek, in Koine Greek!]