My advice would be... spend no more than a tenner. Honestly, you have these gaming mice that cost anything up to £100 with all these programmable buttons, but all I've ever had are bog standard mice with 2 buttons and a wheel and occasionally an extra button or two on the side which never get used, and it has always been fine for games. In BF2 I was always first or second for kills at the end of the round, and a lot of the clan members I played with all had epic gaming rigs with all the extras, including ludicrously expensive mice. During my WoW days I was on the guild's A-Team as both tank and healer and every game I play I find the cheap mice totally adequate.
I know it sounds like bragging or whatever but it's not, I'm just saying that gaming mice are an unnecessary thing. They look nice and the flashy colours are all pretty cool and I suppose it's like a mark of "I'm a real gamer" to own one but as far as being good at a game goes, an expensive gaming mice gives you absolutely no advantage over a standard cheapo mouse whatsoever, at least in my experience.