Rav Shimshon Rafael Hirsch (R'ei - 11:30) says that the language (הלא המה) implies that the mountains were already known to the people. He also says that their difference from each other as well as the point that they were actually the first place that Avrohom built an altar could account for this.
Note that as described, one mountain is lush and green and the other is completely bald and desolate. Thus, they signify the blessings and the curses. The kohanim in the middle would turn to Har Grizim and say a blessing and the people would answer amen. They would turn to Har Eival and say a curse and the people would say amen.
Rav Shimshon Rafael Hirsch in R'ei (11:29) points out that they are in the Ephraim range of mountains. Har Gerizim, to the south of the valley of Shechem is a "smiling green slope rising in fruit covered terraces to its summit." Har Eival, on the other hand, is on the north side and is "steep, bare, and bleak."
Also, there is the significance of their being farther west than just at the Jordan. Since they were near Shechem, Bnei Yisrael would have had to travel farther than where they crossed the Jordan to get there. However, the pasuk implies that they were to deliver the blessings and the curses (at the mountains) as soon as they crossed the river.
According to Rav Yehuda in Sota 33b, they were at the actual location near Shechem and Bnei Yisrael had to travel farther west in order to get there. According to Rav Elazar they actually had to build two mounds at the crossing point to symbolize the two mountains. In either case, he is pointing out that the blessings and the curses and the mountains involved are unusual and would require extraordinary actions on the part of Bnei Yisrael.