It really depends on your tasks, so note what memory-hungry apps you run and check Task Manager (ctrl + alt + esc) to monitor both CPU and RAM usage. 2GB is usually the 'sweet spot,' but 3GB is ideal for heavy gamers and multimedia buffs. Despite the hardware being capable of handling 4GB Windows (including both XP and Vista) rarely can address that much, showing usable amounts around 3.5GB or less in most cases. (It's due to the allocation of RAM and influenced by the exact hardware installed.) Thus, unless you have a 64-bit version of Windows, which lacks this limitation, it's generally not worth it to upgrade past 3.5GB, and sometimes 3GB.
As to the graphics card, not something I can comment greatly on, though you may want to check the NVIDIA Control Panel and see how much shared memory it's looking to use.
What's the sweet spot?
With XP I never needed more than 1GB. My new 5600+ PC just arrived last weekend with 2GB and I have 2 empty slots... Given the low cost of RAM I am wondering if upgrading to 3 or 4GB would give a noticeable improvement? Vista is sluggish on occasion but hasn't given me any huge headaches yet.
I read somewhere that there is a chance that the PC won't recognize any more than 3GB although Dell says my system can have 4GB.
I also read some comments over in a gaming forum saying that my GeForce 8600GT is a system memory hog... The card has given me great performance (video editing and mild gaming) but I am wondering if an upgrade would reap benefits there as well?