Alright i would suggest a Core 2 Duo CPU. It will serve you the best with the Photoshop stuff you want to do. It will also serve you well in games that you do run.
It isnt that XP can run only 3.5GB of RAM. It is 32 Bit Operating Systems. Also known as x86. There is a limitation with them. All explained here:
Difference Between 32 Bit (x86) and 64 Bit (x64)
Plus most places dont even carry XP anymore since it is no longer to be sold by OEM (Original Euqipment manufacturer's) anymore as of July 1, 2008. So most likely you will have to get Vista pre-installed. I would check to make sure that the stuff you want to run is 64 Bit compatible as Vista 64 Bit would be the way to go.
As for the RAM. 2GB with Vista is just fine. 4GB would be better for the extensive stuff that you will run. Vegas, Photoshop and that are really heavy apps.
I would look at a Dell, HP or Asus.