Don't worry I got ya.
PCIE 2.0 to 3.0 will not affect the performance of your GPU. Your GPU or pretty much any other one out there atm doesn't make use of the extra bandwidth that 2.0 to 3.0 offers (to be sure to be sure x8 to x16 same biz).
Going to the Haswell you will see improvement in games that are coded/weighted/intensive to CPU. A lot of game logic going on, which I think does apply to the games you play. Your rendering and smoothness in graphics however will remain the same with the same GPU. So to summarise if you go to the trouble of a new motherboard and a new CPU, your gaming will load quicker but will generally look the same. Not really worth it since you're getting close to max as it is. IMO your CPU is still good to go.