Depends on what they'll give you for it. They gave me $100 for my old PS2 (big model), 2 controllers and memory card when I got my PS3. I did this because I the PS3 played PS2 games and was so damn expensive plus with the popularity of the PS2, I knew it'd be around so long that if I wanted another I could eventually get one for $50 or less.
I'd say if they don't give you at least $120 (2 new 360 games) then keep it unless:
1. It is an older, larger model
2. It's about to go kaputsky because of laser issues
3. You're getting a PS3
The PS2 has such a large catalog of good games that even with graphics inferior to the 360 there will still be some fun ones.