There is a website for the 3 g switch that NickN kindly suggests its here:
http://www.prophotowiki.com/w/index.php/IncreaseUserVa
and worth a look before doing the switch
volunteer
you must be careful of the variable used. If you do not understand how to fine tune that yourself by taking into account the address space the video card also wants to suck off the system, I suggest you leave it at the value I posted above. That website refers to values well above what I posted, which can be done, but not with a 768MB+ video card in the system