I like my Lian-Li PC-C37B. There are many other nice low profile cases out there. Some of them only fit Micro-ATX form factor, so don't try to jam a full ATX motherboard into them.
Also, be careful with these cases only fit low profile cards. Some video cards and TV tuner cards have a short board, and come with both a normal height bracket and a low profile bracket, but you have to shop for them in particular.
The Core i3's integrated video is quite capable of HTPC duties, but its 24p is slightly off the ideal 23.976 fps most Blu-rays are encoded with. Only you can answer if a frame drop every 5 minutes is within your viewing tolerances with its 23.973 fps, or a duplicate frame every 42 seconds if you set the refresh rate to 24.0 fps. If you are going to use a discrete graphics card instead of the Core i3's integrated video, there are some AMD 6450 and 6570 cards that are low profile. I'd recommend going with the integrated video first, and if you don't like it, add an AMD graphics card.