Just one, possibly useless, tip (I haven't had time to review this thread fully, perhaps tomorrow)...
If you decide to go with a discreet GPU instead of bumping up the CPU (and I'm not really commenting one way or the other on that front with this), you might want to consider an AMD card if power is your main concern and if the machine will spend a lot of time at idle with the screen off. My HTPC certainly does.
Current AMD GPUs (the 7000 series) have a nice feature where when the monitor is off, they
completely power down the GPU. It makes a fairly dramatic difference in idle power with the monitor sleeping. On the other hand, if it will spend a fair amount of time at idle or low-power mode (doing simple 2D things), but will be shut completely down when not being used (or put into hibernate mode), then it washes out a lot more. This has a much bigger impact with the larger, higher-end GPUs, of course.
However, also be wary of Nvidia's low-end lines. The
640 is the lowest-end card using the current silicon generation (and the 640 isn't a particularly good value). If you go with an older model (or a new "model" which is really just a rebranded old model), you could end up with much, much higher power consumption and/or noise.
That all said, I don't know if it'd be worth it overall to do that swap (cheap GPU instead of better CPU). Depends on pricing and overall goals.