Your best bet would be to get a router which goes between your modem and computer/PS3. These have 4 ports on so you could connect your computer to one and PS3 to another. For a few more quid you could get a wirless router then you don't need to run a cable from the PS3.
Something like this would be suitable
http://cgi.ebay.co.uk/Wireless-N-802...item35a5aafa6e
Or this
http://cgi.ebay.co.uk/Netgear-WGR614...item53de1d7f77
If you don't want to use Ebay you could pick one up from
www.ebuyer.co.uk or from a local Maplin. Make sure to get a cable router not ADSL.