It's very hard to say. If you have a figure you think it's worth then don't forget you can put a reserve price on the auction.
For what it's worth, I sold my PS2 around this time last year via ebay and I got �74 for it. It was the original model, boxed, in great condition and the extras were: an rgb scart lead, a region X dvd disc, a dvd remote control, a memory card and an assortment of playable demo discs.
As you have a few games with yours and demand for the PS2 hasn't really dropped much since last year (I think!) that should hopefully give you the kind of figure to expect for it.
Have you considered selling the 18 rated games seperate? Just a thought, but if someone is looking to buy a PS2 for their kids as a Christmas present they may be put off by those games.