Ok makes sense - you just have a modem. Well you will need some time of router to divide that signal you are getting so both pc and ps3 share it. If you say that the PS3 is only a few feet away then just used a wired ethernet cable a few feet in length then - no bother, i thought you meant different rooms etc. Also wireless will be fine for you if its in same room, no thick walls inbetween
Homeplugs are great and so little is known about them in this country. Basically they route data traffic over the existing power supply in your house. For example - router downstairs - put one home plug in down there - then lets say you have a office upstairs - you just plug the other homeplug in power socket up there with cable going to either PS3/PC whatever - is easy neat and affective. In your case though you would not need these. Just get a wireless router i would say is best.