Technically a 32bit windows can utilize 4gb of ram, but you have to substract whatever is on the graphics card and other things from that (I'm not sure exactly what, but the graphic memory doesnt seem to account for it all?).
You typically end up in the 2,5-3,3gb area from what i have seen.
That being said, if you bought dual lane RAM its not a complete waste, since it will work better and have dual lane speed all the way to the limit. (if you had one 1gb + one 2gb sticks, you could only have dual lane untill 2 gb, if at all)