You would probably regret getting a Belkin router, they're not really good IMO. You should stick with that Buffalo and flash it to the DD-WRT firmware and it will give you enterprise features for $35 and much better performance.
Shouldn't have a problem upstairs but if you strategically place the router you should get a good signal from the second floor. But if the signal isn't good enough you can buy another of that Buffalo using the DD-WRT firmware and use the second Buffalo as a Wireless LAN repeater.
Two of those Buffalo should be enough to cover a large house. Just try with one first and see how it goes. I can't really say for sure because everyone's house is different and react differently to the wireless signal.
But first before you purchase anything, check the laptop and see what wireless adapters specification are on them. You want it to be 802.11 b or g. If it's both that's fine too, that just mean it's a 802.11g since these two standard are practically compatible. But if it's just 802.11b it will still work but only at 11Mbps or lower speed. The actual speed is less than that, probably 2.2 or 5Mbps. It's best to have 802.11g since you have a max speed of 54Mbps.
