What happens is quite normal with wireless. Concrete is the culprit here. It is best to place the router/anetenna in a central place in the house, a lot of people use their staircase, because there are no concrete walls/ceilings that separate the floors. The signal can roam here. Advantage of this is that your wireless signal is directed inwards (that's where you want it ) which is safer (on the street warddrivers are active...)
You also might consider buying a stronger new antenna, preferably with a (max!!!!) 1 m. wire so you can place router and antenna in different places (staircase?). This will increase the signal some more.
By the way: sorry to inform you that Intel Wireless is not recommended. With NIC's they are very good but when it comes to wireless they stink. You might consider an additional PMCIA or USB wireless card with a Broadcom chip (Asus also uses Broadcom)