I've just installed Photoshop CS4 on my little Acer Aspire One, which has been pimped out with 1.5gb of RAM.
Even then, the adobe installer said I should have 2gb and 1024x768 in order to use it properly, but the program still runs.
I'd make sure the screen is at least 720 pixels high, I think the ideal machine would be a 13" MBP with 4gb of RAM... I've used a friends MBP and it's a lot faster than my Netbook with larger files...