I'd suggest trying both combinations, with and without the single core machines. The fact that they have 2GB of RAM suggests that their single cores are new enough to be fairly fast.

Vue 6 Infinite (I haven't tried network rendering yet with 7) divides each frame into a bunch of tiles. It sends everything needed for the render to each machine - so gigabit networking is a big plus since textures can get very large - and then assigns each machine a tile. As each tile is done, it retrieves the result and assigns the machine another tile. At the end, it will assign any tiles that the slow machines haven't completed to idle machines in the hope that the fast ones can beat the slow one.

So if the slow machines can produce a couple of tiles each per frame you can win with them included.