The performance gains that you will see by replacing those hard drives depends on which hard drives you replace them with. The best performing 7200RPM platter drives available are members of the Western Digital Caviar Black lineup. I have a total of 6 of these in my system in two separate RAIDs. 2 1TB are internal and are connected via the Intel PCH. 4 2TB are in an external RAID enclosure and connected via eSATA. The 8TB external RAID nearly saturates the eSATA connection (SATA II) with read/write speeds approaching 300MBps. As advantageous as this may seem, I don't think that its a real help unless you're editing photos received from the Hubble Space Telescope.
Before you go about trying to improve performance in Photoshop and Lightroom, you should first figure out whether or not changing up your storage mediums will actually improve it at all. I can see how having a better storage backend would improve video editing, but I don't think that it would improve singular photo editing.