Better say try a card with more than 2gb!
1.5 perhaps won't make you happy.
Concerning 2.5k or even other cpu: If a cpu is too slow, it bottlenecks, but a 2500 k doesn't need to be running on 4,2ghz as it perhaps gains 1 or 2 more fps in clod in comparison to tweaking it to 3.3 or up to 3.7. From then on, the cpu should not bottleneck and mor cpu power do not gain much more fps. To gain fps and performance, better buy a video card with more than just 1 gb. I would say, that 2gb is minimum in my point of view!
all additional stuff helps you stabilizing like better ram or an ssd drive, as streaming engines are performing better, when u use ssd. Even your general experience of your whole pc will invrease.