I have both a 5400 RPM and a 7200 RPM HDD in my system. The 7200 RPM is 1TB and the 5400 is older and 500 GB. The difference is really minimal, tbh look at some reviews of read and write speeds, and you won't find a huge difference.
Plus, more space> speed, especially when you are comparing 320GB and 1TB. simply not worth it to get a 320GB, I guess you could get a SSD..