It's hard to say because Piledriver isn't even out yet, it won't be out until October. Until then we don't know how it will perform and if it will be worth upgrading.
a ph ii x4 840 won't be bottlenecking a 550ti or vice versa. they're decent match for each other. even getting a pd 8 core won't improve gaming perf as in this case, the 550ti would be the limiting factor.