Stupid Memory question

I have an EVGA 780i.

In the old days you always had to add RAM 2 sticks at a time. I see people with 3 gb now. So can I add 1 more 1 gb stick to my 2gb?

I know that XP cannot use 4 gb, but wondering about 3gb .

When I upgrade to Windows 7 then I'll go 4.

THANKS!

XIp