I want to buy Windows 7, but I don't know what version would be ok for using softwares like Sony Vegas, Adobe Photoshop, Adobe AfterEffects... I suppose it doesn't matter?

A better idea is to check and see what software OS is fully supported by the editing program.

In Sony Vegas System Requirements are just Windows 7. And Adobe says they tested their product on Windows 7 Ultimate...

Most pro video software states Win 7 Pro as a minimum (eg. Matrox). Also, Win 7 Home Premium has artificial limits such as a max of 16GB of ram whereas 7 Pro does not.

Another good reason to get Pro is XP Mode, which is a virtual machine that enables you to use older software. I use it for non-professional programs so they don't interfere/cause problems with my expensive professional programs. (XP Mode is free but only available with 7 Pro & 7 Ultimate)