Right. The poster above is right about cellars and wireless. I suspect that the best way forward would be to move the cable modem upstairs to where the cable inlet is, and attach a wireless router to the cable modem. Since wireless routers have a number of wired network points as well, you can either connect your downstairs PC to the router wired or wirelessly - there is a lead running from the inlet to the PC already - yes? - so replace that run of cable with a network cable instead of coax, or you need to add wireless capability to the PC if it doesn't have it already - USB sticks are available for this purpose.
You could then connect your PS3 to the router wired, since I am presuming that a) the PS3 is by the telly and b) if the PS3 gets its updates off the internet, it could do it wired or wireless, and wired is painless.
Then it's just a case of getting any wireless devices to talk to the router, which is generally the tricky bit. It will be useful to connect your laptop to the router using a network cable for configuration purposes, setting up security and passwords etc - get it working wired before you try wireless. Follow the instructions that come with the router religiously, including the order in which you turn things off and on, and leaving them off for periods of time.
As for which router you choose, I have a 'G' router that looks like
this, but any wireless router should be fine - going 'N' may be a risk since the standard isn't agreed yet, others may comment on its compatibility (or lack of). What you do not need, by the sound of it, is a combined router/cable modem, since you have one of those already, though you may wish to reduce the number of boxes by getting a combined one, at greater expense, obviously.
Don't worry baby I'm safe and sound.
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