I thought I remember seeing a thread in here a while ago asking the same question. I have a feeling the answer was that you will have to completely wipe and reinstall Vista to go from 32-bit to 64-bit.
The search for "upgrade vista from 32 to 64" gave me the following link:
http://windowshelp.microsoft.com/Win...c8a701033.mspx
Sadly it's what I mentioned above. Oh, and btw, you have an Nvidia 8800GTSI presume. ATI never made an 8800, or at least not one that could run Crysis. It would be about 6 years old now, or more. :P
About the RAM, you probably have 4 sticks of DDR2 RAM already, and since you only use a 32 bit OS, it can't fully utilize all 4GB. Keep it the way it is, unless you feel the need to go to 8GB. No need as of right now, unless you have the monies. The 8800 is good, and unless your PSU can handle it, I could suggest upgrading to the GTX280 or so, but that's overkill unless to use super high resolutions (beyond 1680x1050). Also, you may want to post your entire dxdiag to see exactly what you have in your rig. The more info the better.