As looking at your current configuration, I can suggest you the following. Since you don't have any specific OpenGL card now, I don't think you should get one now either. And you don't play games. Photoshop loves RAM, so lets see:
Motherboard:
A8N-VM CSM $82
http://www.newegg.com/Product/Produc...82E16813131570
-Integrated everything. Stable.
Harddrive:
Western Digital WD2500SD 250G $105
http://www.newegg.com/Product/Produc...82E16822144179
-Resonable amount of space, reliable. 5y warranty, certified for 24/7 use (other cheaper drives certify only 8h/day)
Memory:
2xBUFFALO 1GB 184-Pin DDR SDRAM DDR 400 $74.55
http://www.newegg.com/Product/Produc...82E16820150007
-Photoshot and CAD loves memory, here is 2G of CAS2.5 (a tiny bit faster than CAS3) of it
Ok, so now we're at $336
CPU:
AMD Athlon 64 3200+ Venice $164
http://www.newegg.com/Product/Produc...82E16819103535
Get's you to the total $500.
You can skimp on the memory to save the 74.55. You can survive with 1G.
Case & PSU:
Recycle from old computer. Along with the rest.