The standard right now is ATX form factor. ATX motherboards have 7 expansion slots. There's also micro ATX, which has 4 expansion slots. (it's longer)
A micro-ATX motherboard will fit an ATX case, but for obvious reasons an ATX motherboard will not fit in a Micro-ATX case.
Pretty much any good case you get will be ATX....
OS- Windows 7 Home Premium Retail- £95 (OEM was only 20 cheaper..)
CPU- Intel Core i7 860- £223
Mobo- Gigabyte GA-P55-UD4 iP55- £137
GPU- ATI Radeon HD5850- £250
RAM- G-Skill Ripjaws 2x2GB- £95
Case- NZXT M59- £48
PSU- Corsair 850W TX Series- £100
Hard Drive- Samsung Spinpoint F3 1TB- £60
DVD Burner- LiteOn IHAS124-19- £18
If you want to save a bit, change the cpu to a Core i5 750.
It'll be about the same in gaming, but slower in video encoding. (although, if you're using Adobe software to do the video, the 5850 can handle the workload anyway)