It's been awhile since I've really looked at the newer CPU's and I'm thinking about building a HTPC. I'd like some recommendations on the most economical CPU and MB to use. I think I've read that some of the newer Intel CPU's have a decent GPU included. I would like to put something together cheaply that I could use primarily for web surfing and yahoo email but wouldn't mind having enough power to possibly be able to add a video card later if I wanted to do a little gaming and watching video. I'm also thinking about using a Linux Operating System as I've wanted to try it and this might be a good time.
Generally speaking, you want something quiet as the last thing you want to hear when watching a movie is the whirring of fans. Quiet for a HTPC means low heat, so you're wanting to look at CPUs with a TDP of 65W or less ideally. The i3-2100 is a good bet for that. There's also the i3-2100T, which is really just the i3-2100 that's been underclocked a bit and allows you to get a TDP of 35W. There's not much of a price difference between the two. There's also the Pentium G620 / G620T chips...S1155, cheaper versions of the i3 CPUs, but not so good at transcoding. They come in 65W and 35W flavours, respectively, too. I can't comment on how it performs, but I've taken delivery of a G620 today as my research suggests it's a great HTPC CPU.
The problem with an Intel chip for an HTPC is that it can't play Bluray at 23.976 fps, which is what it's filmed at...it can only manage 24 fps. So you get stutter. An inexpensive GPU can sort this...the AMD 6450 is rated highly for 23.976 fps (it doesn't quite manage it exactly, but is just about spot on). The Nvidia GT430 (I think?) is also a good option.
Another option you have is the AMD Llano CPU. Right now the available chips have 100W TDPs, but the 65W variants should be shipping soon. They have AMD 6xxx GPUs onboard, better than the 6450, and you wouldn't need a separate GPU.
If you decide to go with the integrated Intel (or Llano) graphics, get a motherboard that supports HDMI out. MicroATX or MiniITX is a good size, so you can have a relatively small case in your lounge. An HTPC doesn't need a motherboard with all the bells and whistles for overclocking etc etc, so I wouldn't spend more than £50 - £65 on one.