We calculate the microstates free entropy dimension of natural generators in an amalgamated free product of certain von Neumann algebras, with amalgamation over a hyperfinite subalgebra. In particular, some `exotic' Popa algebra generators of free group factors are shown to have the expected free entropy dimension. We also show that microstates and non--microstates free entropy dimension agree for generating sets of many groups. In the appendix by Wolfgang Lueck, the first L^2-Betti number for certain amalgamated free products of groups is calculated.

The second revised version significantly generalized the main result of the original one, (see the abstract) and contains a new appendix by Wolfgang Lueck. The third and fourth revisions correct some minor mistakes. The fifth version adds a result about embeddability of amalgmated free products