Well, I always like using the Mid-Towers...usually have plenty of room to work and lots of extra spots for drives. Best bet is to get a 300w power supply in it. Usually under 90 bucks for a nice one.
Motherboard...well, pick a chip and then find the board that works for it. You can go crazy with some boards and get them with RAID support & all. I used to hate the integrated sound and all but now I'll build on the integrated sound and integrated network. Just make sure you DO NOT GET A MOBO WITH INTEGRATED VIDEO. These usually don't have AGP slots. You also want one with 2x4x8x AGP.
I've been very happy with the 9500 pro video card I got last week. All games I could not play before I can now...BF1942, Jedi Outcast. I can even put the smoke nades on slowest, best looking and have no lag!!!
Hope that helps some...I'm sure if you post the chip you want here someone will suggest a good mobo. Also, I've used Gigabyte and Asus in the past with good luck.