Actually.. now I'm wishing I had gone ahead and spent the extra for the quad and the vidcard with 1 GB...lol
Climb down, come back in, everything is okay. Trust me, I never lie, and I'm always right.
Unless you play graphic intensive games, or do 3D rendering, (Bryce etc) you would not see any difference in performance.
In Photoshop CS4 and CS5 on systems with over 256M of VRAM the GPU is used for smoother zooming in and out, rotating the canvas, displaying and working with 3D objects, and color correction among other things. Here's what GPU acceleration does
in PS. You're getting the acceleration, and the extra VRAM would not make any difference unless maybe
you were applying any of the effects to multiple pics at at once.
Going from 4 to 16G of system RAM is the biggest improvement many times over as compared to more VRAM anyway.
You'll want to tweak Photoshop. It takes 70% of system RAM by default, you know where to change that. You'll want to balance out what it uses and what other apps need, you don't want PS sucking down RAM it doesn't have to. Set it to 8G at first. Do a normal day or two of work, then doing "top" in Terminal check out the free RAM. If you have plenty of RAM still left you can allot more to Photoshop.
If you only have a few hundred MB of RAM free it may be a sign that PS is taking too much and starving other apps.
If it says you have plenty of RAM free, but there are 100's of thousands of pageouts, PS may need more. It will take a while but watching top and tweaking PS will get you to a good balance. My guess is for what you do 8G will be enough for PS to be set at though.
Also for reference you can use the Activity Monitor app to check out the RSIZE, sort by RSIZE, real size, to see how much apps are actually using.