Hi,
If I remember correctly Photoshop in CS4 is still 32 Bit so you won't gain from increasing your RAM. CS5 has a 64 bit version of Photoshop and this allows you to use more RAM. Using more RAM in Photoshop or other 64 Bit Programmes does help things along. For example, rendering an image in Photoshop that used to take several minutes when I had 4 GB of RAM now takes seconds.