I am looking at upgrading to a new system. I will be mainly playing 1st person shooters but also some virtual servers. Should I buy an Ivy Bridge CPU or wait for a Haswell CPU?
This is tricky, but I still recommend Ivy Bridge. Sure, when Intel release Haswell it'll bench better than its predecessors and the competition - but will the cost involved be worth it? Maybe.
If you were playing with a multi-GPU, multi-monitor setup - most likely. The CPU bottleneck would be removed some what with the extra grunt, but again, by how much? 5-10% maybe? 15-20% would be my guess on a high-end, multi-GPU, multi-monitor gaming rig in say, Battlefield 3. But then, you'd not be worrying and just buying it when it hits.
I'd recommend getting an Intel Core i7 3770K, that'll see you through the next year or two at least. You can overclock them easily to 4.5GHz, which will remove most bottlenecks from a single-GPU rig, too.