Well postaldudeleo you should really take a look at what you are writing here... Let the Geforce use 2gb ram on its own, no way. There is no way you can adress the RAM to use for graphics (unless you are thinking of APG Aperture Size but thats bull**** with 2gb on that one, sooo unecessary).
And I kinda figuread that you meant you had 2.25gb RAM (OCZ brand, yes good ram I agree on that) and a Geforce 6800 Ultra (that has 512mb RAM).
But lets get into the 4gb RAM discussion here... That is not needed as windows can't adress more than 3576mb RAM meaning that whats over will be "backup temp storage" for PIA units and the like...
Also when having 2.25gb ram I figure that would be... hmmm... 2x1gb sticks + 1x256mb stick.. That would kill a lot of the system as it would disable Dual-Channel...
Anyway back on topic here... Get the crackfix and try that one, or play with your original disc. If it still crashes I would say that some driver is crap or that some of your hardware is failing in you!