Could it be that you bought Ram from diffirent Companys, so that the Timings and the Latency is diffirent to your old Ram?
Only the lowest Settings will be used with two diffirent modules, be sure that you not build it in in dual channel mode, check the Manual of you Mainboard for that.

If you use a 32-Bit System it will show you something about 3-3,5gb of Ram cause it can´t work with more than 4gb but also needs some Memory Addresses which are included in the 4gb,
with Vista 64bit you can use more than 4gb, but to really take effect you should build in 8gb of ram indeed, cause Vista 64bit need more Ressources.


Hope i got that all right, need a little more practice in englisch
have fun