I've the same machine as the original thread (just NVIDIA GeForce GT 120, 512Mb).
...
- I use this machine mainly for photography (no video, no games) so photographic softwares (CaptureOne, LR, PS).
Depending upon where tracking latest PS or committed to "older" PS because it is good enough, there can be some upside to moving to a GPU that supports modern OpenCL.
http://www.tomshardware.com/reviews/photoshop-cs6-gimp-aftershot-pro,3208-6.html
It isn't going to give across the board speed ups but if make high usage out of accelerated ( OpenGL and OpenCL ) PS subcomponents, there is real speed improvements. Leveraging OpenCL just started with CS 6... given the need to show substantive improvements with the subscription model Adobe is very unlikely to slow down further transitions to OpenCL of a wider variety of effects.
If you are not going to track Adobe's likely improvements then the GPU would be low priority.
- I only have 7200rpm internal drives (Raid1 for data, not the OS)
An SSD for OS/App/Home directory is likely to have across the board improvements across all applications and general usage.
Whether you want to go
SSD : OS /Apps/Home dir
SSD : working space ( temp and scatch space files )
RAID-1 : long term data
depends upon budget, but that would be substantial speed up.
( 1 OWC Electra 3G 256 GB + 2.5"->3.5" adapter $250 OS/Apps
1 Samsung 840 Pro 256 GB + 2.5"->3.5" adapter $330 Working Space
For an Apps drive the OWC Sanforce drive is fine (and stay away from enabling TRIM hack for it). For high density of jpeg and compress image files, it is not so much the top option. )
- Drive W/R seems to be always at maximum level
See above. Spreading out the different classes of data over more devices will help.
- RAM gets close to 90% often
The OS is going to soak up RAM ( e.g., as a file system cache of getting data off the HDDs and into RAM). The problem indicator is more so the Page in : Page out ratio and the swap usage (ratio of RAM:"Swap usage" ratio to be high. Swap small and controlled ).
Relatively high swap usage and/or relatively high page outs are indicator of a problem. Activity Monitor will indicate what you usages of these are in the "System Memory" tab.
That said if tracking the latest OS upgrades 10.8 and up with 64-bit only kernels pretty much assume folks will have at 8GB of RAM to do anything. So if running larger memory workload. If you Photoshop and LR sessions are using 3-4 GB of real RAM each then probably would usage to move up 12-16GB of RAM. eight 2GB DIMMs is only about $146.