CS4 doesn't run as well as CS3. I have 4 gigs of RAM and both versions and usually use CS3, unless I'm editing RAW files, using the clone stamp, or feel like using tabbed windows.
Upgrading your RAM couldn't hurt, but the recommended system requirement is 1 gig for CS4. Personally, I'd just buy more RAM, but I'm sure 2 gigs is fine. You can always upgrade in the future if it's too choppy. There's also the option of disabling open GL, changing Cache size ect. I assume your processor is adequate since you have DDR3?
I am thinking about getting an iMac and photoshop cs4, and am wondering if i should upgrade to 4 gb of ddr3 or if i can stick with 2gb and still be able to multi-task a little while using photoshop? (by multi-tasking, i mean something like having a firefox window and skype open)
thanks in advance