What's the most suitable GPU for this system?

[bruce]

By default, the FAHClient will create a slot to process CPU assignments and another slot to process GPU assignments. You can expect that one CPU thread will be use (at 100%) to support your nVidia GPU, though it may not look like it since the Windows job scheduler moves that load from one thread to another randomly. Almost any GPU from NVidia (plus the one support thread) will accomplish a lot more science than your CPU so it will earn more points.

The balance between the GPU and the CPU depends on the slider position but there are a lot of other options that are ignored by the slider. If you use FAHControl you can change those settings but in most cases, the slot's FULL setting is fine unless you have a problem dissipating the heat.