How to?

(1) I would buy an OEM disc of whichever flavor of Vista you want for two reasons: The First is that upgrades always leave behind garbage from the original OS and therefore clean installs are always better unless you have some specific reason to do otherwise. The second reason is with a complete disc you'll not have to keep your XP discs.... Make that three reasons - You can still continue to use your old computer (legally licenced).

Regarding the 4GB of RAM - There is a bug involving certain types of memory controller. There is a fix - KB929777 - and this may or may not apply to your particular components. Irregardless, Service Pack 1 includes the fix. You'll probably be fine, but read the stickys in the OCZ forums regarding running 4 DIMMS (if applicable), and as also recommended there you may want to manually set your tREF to 54 or greater. And if you want to be sure, install with 2GB, and add the other two later.

Regarding Vista 64 - The Kernel of Vista 64 is completely different from the 32 bit version. You can NOT 'upgrade' from 32 to 64 bit operating systems. You must do a clean install. So if 64 bits is your intention/goal/thought, you would be better off if you bit the bullet and simply went for it. The alternative is to have to back up all you data and load operating systems and all of your programs twice.

Hope that helps.

Single is what you want...