A dedicated HTPC (inc a streamer) built yourself will always be a better option for quality, pound for pound, than the likes of the WD Live Hub you have at the moment.
Depending on budget and preferences a device that will not drop frames and render in full 1080p is easily achievable for any budget from £150 upwards.
From my experience, I would offer the following suggestions based on your short list (sorry to object!):
- Go for an AMD A series chip over an Intel Core i3; whilst Intel's Core i3/ i5 and i7 chips are hands down better than anything AMD can throw at them at the moment in terms of benchmark tests and multithreading capabilities, AMD's strong point is their integrated GPUs. I have an AMD A6-5400 (£45) in my HTPC and have no issue streaming 1080p from my home server over a wired connection. It never drops frames, colour bands or pixelates. For reference, I play through a Samsuing Series 6 37'' TV.
- GPU; See above and save your cash.
- RAM; Depending on your OS, I wouldn't advise spending money on more than 4GB. You won't use it as you'll be caching so little when playing media.
- Storage; If possible, go for a dedicated SSD for the OS and store media on an HDD. Make sure the HDD is at least 7200rpm and you'll be fine. Again, depending on the OS, you could even run it from a 16GB USB stick.
There are a number of things to consider which will better help to focus you on what it is you want:
Think of your list of priorities beyond excellent picture quality and good amounts of storage for media as these 2 things are obviously a given. Decide if it's more important to you to have a case that looks the part and if so what style? Is it important to you to have something that boots and shuts down as quick as a BluRay player does? Are you going to be doing anything else with the HTPC? If so, then are you going for Windows or Linux? Both have their advantages.
Personally, in my case I wanted a fast to boot and shut down HTPC that would be silent without serving any other function apart from playing media over my network. I built a fanless PC using OpenElec (A well supported XBMC Linux build that I can't recommend enough) that boots to the XBMC homepage from powered off in less than 5 secs.
Keep me posted with what you want and I'll help you come up with what would suit you best.