How can I check what’s the maximum RAM supported by my system?
The Crucial web site will usually also tell you the maximum amount of memory supported by your motherboard.
free scan of the computer at Crucial
How much do you need
After writing down the model number of your motherboard, visit the manufacturer's website and search for your motherboard model number. You system specs will explain the ram it can support
Memory Limits for Windows Releases
As far as 32-bit Vista is concerned, the world ends at 4,096 megabytes