While RAM disks may seem like a throw-back to pre-OSX days, 10.5 and below are still 32-bit and as such force Photoshop (even CS4) to a 3 GB RAM memory limit, no matter how much glorious RAM you have installed. And, because you should set the RAM limit usage to no more than 85% (it's already that or less by default), you don't even have access to 3 GB. (Behind the scenes PS is using some additional memory for plug-ins, but about 2.7 GB is really the max until Snow Leopard comes out and PS CS4 is rewritten for it.) But if you do have lots of RAM, then use this app to create a nice, big RAM Disk (maximum is 2 GB under 10.5) by holding down the option key at start-up. Then, in PS prefs, under Performance, set the RAM Disk to be the first Scratch Disk, move it up to the top of the list with the up-arrow key (on the right side) and now if PS does need to "go out to disk" it's going to memory! (Keep in mind this is not where you're reading and writing files to and from, it's just scratch for PS, nothing more.) Set it to always start-up with it in your login items and it'll always be available to Photoshop as a drive.