i have already upgraded my rig , and i want my 4GB ram to be recognized
This is not a limitation of Vista. It is a limitation of any 32 bit operating system. The short answer why is that the Operating System simply doesn't have enough address space to give all 4GB an address.
If you want to see a display of all 4BG, then you will have to go with a 64 bit OS. The downside of that approach is not all vendors have 64 bit drivers for their hardware, and there's a good chance you'll have troubles running older games and apps. This is why I suggested the additional HDD/Dual Boot approach.
Why would you dump your XP at all? Vista 64bit has issues as all Vista versions do and XP is more stable even if there are still issues. I think Scott had the best suggestion. I'd keep the XP as a back up in case there is something that you have trouble with in your Vista OS especially if you go with 64bit.
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