For reference, here is my current HTPC setup:
MOBO: Asus P7H55-M Pro
CPU/GPU: Intel Core i5-650
RAM: Corsair XMS3 4GB DDR3-1600
PSU: Antec Earthwatts EA-380D
Case: Lian-Li PC-C37B
SSD: OCZ Vertex 2 90GB
HDD: Seagate Barracuda 1.5TB
Some of those items are obsolete (the build is a little over a year old), but that will give you an idea on what specs I'm running. The machine can handle anything I throw at it. Here are some of my thoughts...
MOBO: I've always been a fan of Asus. I've used them in about 7 builds without issue. That's not to say other options are no good, I just don't have experience with them.
CPU: As I said in a previous post, I prefer intel for HTPC use. They cost a bit more, but they run cooler and quieter. This was a huge point for me as I don't want to hear fans humming away why I'm watching the TV. My current build is using the stock CPU cooler and I believe the case has one or two fans. The whole setup is virtually silent.
RAM: Probably doesn't make a huge difference. I've had good luck with Corsair so that's what I went with. 4GB, however, should be more than sufficient.
PSU: Good choice on that. This also runs cool and quiet. Even if you do an "upgraded" build, I wouldn't suggest anything higher than the 380W PSU. You aren't running a dedicated GPU, so there really isn't anything that will draw enough power to make it necessary. Save a few bucks there.
Case: Mine was expensive. But I love it. It was tough to shoe-horn everything in, and I spent a couple of hours on cable management within to ensure good air flow and accessibility of components. Don't skimp on this step. I'm sure the case you selected would be fine, but spend the time when putting things together to zip tie cables together. It will make your life easier if you need to get into the case to replace components down the road AND help circulate air to keep things cool.
SSD: Definitely not needed. I've got an SSD in every PC I own, so I've grown used to the reduced load times. If you're planning on leaving the HTPC running most of the time, it wont really help as much. If you've got the money, I would recommend it, but you can just as easily save the cash and run everything off of the HDD. This is always something that can be added down the road if you don't mind re-installing the OS and other software.
HDD: You should be fine going with whatever is relatively inexpensive. Sticking with a 5900RPM drive will keep heat and noise down. Especially if you're running an SSD for the OS and software, you wont need a higher RPM drive for the read/write speeds.
Remote: I strongly recommend getting a cheap HTPC remote. I don't remember what model I have, but something like this
should work fine. Once in XBMC, I can control everything using the IR remote. There is even a piece of software that can use a remote button to launch XBMC. I've found this handy as I very rarely need to use a mouse or keyboard with my setup.
I am currently running Win7 Pro on my HTPC. XBMC is then run on top of that. Again, XBMC is free so it wont cost you any more to go this route. I've experimented with nearly all options in the HTPC software realm and found XBMC to be the best (in my opinion). However, there is no perfect piece of software. XBMC is more universal in what formats it will accept. It also does an awesome job cataloging TV shows, movies and music. There are a number of add-ons (also free) that can increase functionality in some areas. The main downfall to XBMC is streaming. There really isn't a good way to stream Netflix, Hulu, etc at this point. I know people have been working on add-ons, so that may change in the future. For now, if I wanted to use any of those, I would just back out of XBMC and run them through their software in Windows.
That's about all I can think of for now. If you have any other questions, post them up. Also, if you get to the build and need help selecting/configuring software, feel free to ask. I've experimented with a lot, so if you hit any snags I would bet I've probably run into the same issues.