Replacing my Iyonix with a different low enegy system (with a poorer gui) isn't going to help the environment.
The second computer would only be powered up as needed, (just as my current second computer is), but would probably boot somewhat quicker meaning it would be less likely to be left on to avoid delays, it also would save a screen.
Thin client applications are very routinely used by full fat client.
Having Open Office run that way is unlikly to be much worse aesthetically than RISC OS firefox and is likely to be better than the earlier versions. Certainly better than having to go through a second GUI in a window or turn to a second screen.
Crossover is reasonable aesthetically, however people use VMware fusion which is poorly integrated rather than use a second PC, and how would it be any worse than running things via X on OSX (eg OpenOffice)?