I know I’m always taunting you Mac users about your lack of 64-bit (I develop primarily on Windows 7 but have a few Macs for various purposes, mostly live shows). But now that CS5 is here you are free to bask in the unadulterated glory of full memory allocation that us Windows users have been enjoying since CS4. John Nack has put together some benchmark numbers that should give you a rough idea of the performance gains you can expect. As Nack points out though, these gains will only be seen by people working with larger files, tasks like web design won’t benefit much. You can check out the numbers here: Photoshop CS5 64-bit Benchmarked
On a side note, my overall experience with 64-bit has been favorable in the graphics realm, but for music I use a 32-bit installation. For music production there isn’t much of a benefit from 64-bit unless you’re doing a lot of sampling, it’s mostly about CPU and disk speed for recording and effects. The DAW software I use to record — Cakewalk Sonar 8.5.3 — still isn’t as stable as I’d like it to be when bridging 32-bit VSTs (many of my favorite VSTs have not been ported to 64-bit native yet) so I’m sticking with 32 for the production of this latest album I’m working on.