Depends on what you want to spend!
Any decent ADSL modem/router and matching PCI adapter and laptop card should give you good signal strength in any normal house (unless you have lead coated wallpaper!)
I use a Belkin F5D7632-4 router and Netgear cards.
Netgear Rangemax kit is expensive but apparently good - DG834GT ADSL router/modem and the WPN511 card/WPN311 PCI adapter. If signal is still weak after all that, throw in the WPN802 access point to boost the signal!
Remember, the whole connection speed thing is limited by the ability of the phone line - not just the claimed wifi speeds