2 extenders on one HTPC

Sure ... I have 3 extenders (2 Xbox 360, 1 Linksys) running off a single HTPC (Core i3). The PC has 6 tuners (Ceton InfiniTV 4, SiliconDust HDHomeRun).

We have 4 extenders and 8 tuners (CetonInfiniTV4 and 4X Magyca ClearQAM)

Not sure what you mean by "less powerful" HTPC does not need to be "powerful" to begin with. Without the extenders, or with just one extender, Intel ATOM Dual core was more than enough to drive live TV from 2x ATI DCT's.

One thing to keep in mind is that Microsoft recommends 1 CPU core and 1 Gb of RAM per extender connected.

If you have TV connected to the HTPC, add another core for it, as well.

With 4 extenders and a TV connected to HTPC, I can run all 5 TVs with 8 Gb of RAM without a hitch.

atom will be a non starter. i have a 2.3ghz core 2 duo and having one extender on tests its limits. i have two extenders, but i cant i dont have any reason to run both and my pc at once.

I'm running dual extenders with a Pentium G630. It's one of the lowest-end Sandy Bridge offerings there is, and it works fine. I believe the Celeron G430 (dual core 2.4GHz SB) is about $50. Add a cheap mobo (just be wary of non-Intel NICs on board), and it's not much more than Atom while offering massively better performance.

Playing on the HTPC does not peg CPU since the video is handled by the GPU. But, with Sandybrdige, I don't know if those two are seprated, or it uses CPU cycles to render video.

Using extender peg CPU, since most of the work is done by the CPU, and only little of the work is actually performed by the extender it self.

I believe WMC supports up to five extenders, IIRC.

This is five extenders at once, yes? You can register more than that, but can only have 5 running at the same time. I haven't hit this max, but thought that was the general rule.

I believe that is the case. I don't think there's a limit as to how many extenders you can connect to the main PC but rather how many it can support simultaneously.