Okay, first, yes, 32-bit operating systems whether Vista or XP won't read more than 3 gigs of RAM.
Second, go to Memory upgrades, flash media, and usb storage at Crucial.com
and allow them to scan your system for free. You'll have to download an Active X but this is harmless. I have used this tool many times to get an accurate read and it is highly relliable. You will not get malware/adware and you are not forced to buy their RAM.
Once scanned it will tell you:
1. The amount of RAM your motherboard will allow.
2. The memory options you have for your ssytem.
Let us know how it goes.