AFAIK the AMD 760G chipset is still the chipset of choice for a HTPC, I built one a couple of months back http://www.tekforum.co.uk/posts/list/12898.page and I’m really happy with it, with the energy efficient Athlon I’m using the stockcooler and I have to be right next to the case to hear it at all, don’t think your going to need 4 gigs of Ram tbh 2 is plenty in mine.

 I use mine to watch DVD’s (I do plan on getting a Bluray drive for it soonish), normal quality downloaded shows and movies, plus HD quality Downloads, I have a DVB-S card in so can watch Freesat, being able to record HD TV stuff at a quality level that I cant tell the difference from the original broadcast is pretty nice as well. I also stream IPlayer and 4oD with it and do the odd bit of surfing and check my mail rather than going to my main comp or firing up my laptop.
I’m debating if to get a DVB-C card for it as well but not made my mind up yet

I have no experience of using a 360 as a HTPC, so I can’t comment directly on how it would compare …but I have heard it doesn’t offer anywhere near the functionality of a old Xbox with XBMC installed, this HTPC beats a old Xbox by a very long way, so suppose it depends on if the 360 will do everything you want it to do or not?

Ignore all Intel chipsets if you want to have a quiet machine.  AMD 780G or 780GX chipsets have full hardware decoding built into the chipset itself.  No need to get an uber CPU to process it as a result, meaning you can get away with a less powerful CPU.  

780G/GX + 4450e + 2Gb of generic RAM will produce the basis of a very good little HTPC.  IIRC Hexus did single cored Sempron V a low end dual core Intel, and the AMD spanked the Intel is all the media tests.  It was designed for HTPC use from the ground up.