Ive got a 40gb ps3 and a bt home hub wireless router how do i connect ps3 to computer ps ps3 is upstairs and pc downstairs

Yes, a wireless router should work fine.
You'll need a wireless-adapter (either internal PCI add-in card, or external USB stick) in your computer, if it does not have built-in wireless capability.

Hello
This not a big deal for a wireless network to work on downstairs, you need to check your connection and settings of the routers/computers. or just put a repeater for improvement of signal strength.
If the matter is signal strength, for the purpose see the TIP link below
http://www.fixya.com/support/r7245572-improve_signal_strength_wireless_network
Good Luck

The upstairs computer is wired or wireless ?

If the upstairs computer is working then the router must be working properly.
Check the connection between the downstairs computer and the router.
If it is a wireless computer then check the wireless connection status on that computer.
Click Here for step by step solution

No. You'll need to run a "cross-over" Ethernet cable from one of the "LAN" ports on the "downstairs" router, connecting to the "WAN" port on another router that you will need to buy, and place "upstairs", and connect the 4 devices to the 4 "LAN" ports.

Or, run a long "straight-through" Ethernet cable from your DSL-modem (or cable-modem) that is "downstairs", and move your wireless router "upstairs", and connect the 4 devices to the router's "LAN" ports.
Your laptop "downstairs" should still be able to connect (wirelessly) to the wireless-router that is "upstairs".

Is your ReplayTV and Xbox configured to use DHCP? If not, you may need to configure static IP addresses for each of them.

Did you use MAC ardress filtering to grant ps3 access to router?
if so you have to do the same with your cpu's wifi mac adress

You would be best to buy a wireless router with a built in modem, this would be much easier for you to setup and manage

if you want to be able to play your ps3 downstairs with your modem upstairs, you might consider buying a Wireless Router that has Wireless G , for your PS3.
This will allow you to access your Internet all around your house without wires. I suggest looking for one that features MIMO or range plus max, or anything else that increases your signal range since there is a fair amount of distance involved.

The alternative is relocating your modem downstairs and hooking it up to the PS3. BUT your computer wont have Internet.

You'll need a way for your computer upstairs to connect to the hub/router. You can most likley get away buying a low priced wireless USB adapter like this one. http://www.newegg.com/Product/Product.aspx?Item=N82E16833340004
If your router has wireless security (such as WEP Encryption) you'll have to know the (WEP) key and SSID of the device in order to connect.

Ok, so why do you need an access point upstairs and a router downstairs connected wirelessly? If you can't connect with a laptop or a pc, why whould you think you could connect the access point? The purpose of this configuration would have to be with the access point to the router via ethernet. What's the point? If the problem is range, check out http://www.dd-wrt.com/dd-wrtv2/index.php to enhance the capabilities of the WRT54G... and maybe building a parabolic antenna for it as well. Plans to do so abound at Lifehacker.com.