You actually have quite a few options/decisions and budget will play a part here too.
Both the PS3 and Sky box can be connected to an av amp via a digital optical cable, this should give you fairly decent sound and it will carry dolby digital ect.
If you go for a fairly new AV amp then it is likely that it will have a form of HDMI switching (please note that this normally only acts as a switch and will not do any processing for audio).
If you decide that you want to take advantage of the new audio formats such as Dolby true-hd then you could consider the Panasonic SA-XR700. This will process the audio from both your devices via the hdmi cable. This is one of the very few amps out there that is capable of this and Im not sure I would recomend wholely, itsnew technology and many more amps will be released later this year bettering the panasonic.
My view is, if your old sony amp can keep you going for the moment then it could be a wise move in the long run. Give it til Sept/Oct and there will be a whole load of new AV recievers offering the latest features.
Or if your feeling flush forget what i just said and get down to Sevenoaks Brighton and pick up an Arcam AVR250 for £595. It wont do any of the fancy stuff but it will sound a hell of a lot better than any other amp for the money