Honestly, there isn't much of a point of waiting for a Mac Mini with a Haswell chip from a performance perspective, it only really matters from a power usage perspective.
There is a model of Haswell chips that has significantly better graphics (the Iris Pro 5200). But those are paired with much more expensive chips, and if you read the reviews, you'll see that graphics like the HD 4600 isn't substantially better.
Furthermore, the areas that the Iris Pro are better than the HDxxxx graphics aren't important for HTPC applications. The primary feature that you may be interested in which came with the Intel graphics features is QuickSync.
So, for your purposes all the Intel chips after the Sandy Bridge line would be indistinguishable from each other for HTPC performance, except when it comes to power efficiency.
So, my advice is if you could use a better HTPC now, get the one on the market you like now - I'm not aware of anything substantially better that is coming in the near term.
When it comes to small form factor PCs, I'm sure there must be units that generate very little noise. When you have a fast chip which is idle most of the time (which it would be for HTPC applications), the chip is sleeping and generating hardly any heat. Just try to avoid models that have really small fans, those are the things that tend to make noise.