HDD will play an important part if your RAM is less like say 512MB or if you have 1GB and have many processes running, there will be HDD swap file usage and game response will be slow. 2GB RAM is the minimum you should be looking at these days.
W.r.t Laptops, I believe that you should get the fastest HDD possible as you are already compromised on speed because of the 2.5 inch form factor(3.5 inch HDD of same r.p.m will give better performance due to larger surface area and hence greater performance in the initial partitions v/s a partition on the 2.5 inch HDD.).
In any case, never get a 4.2K r.p.m laptop HDD even secondhand(I don't think there is any manufacturer that makes these nowadays) as the performance is pathetic and I have experienced the poor performance of such a system combined with an intel celeron proccy.