you need a CPU AND a Mo/Bo ?

what DO you have already ?
I assume you do already have a working system, and that you are wishing to upgrade ?

you say games ?
any games in particular ? some chew up resources more than others

and where have you already tried for input ? saves re-inventing th ewheel as it where .......

I'll gamble that you are using windows XP, 80GB HDD, 512MB DDR RAM, system around 2 years old, and you want to upgrade to keep up with your mates ??

if so, then could you please say where your system is lagging ? not where your mates say ...

Generally, I would start from the Mo/Bo, and work out from there.
if the suggestions start to get too expensive, try and get a quote for a NEW complete system.
just to keep it in focus.........