Question about cat behavior

Question
I currently own two cats (but there are four living in this house all together). My male cat, Sassy, is about 2 years old and my female cat, Kat, is around 9 months old. Both cats are fixed. Most of the time they are the best of friends!. They snuggle together and give each other baths.
Some times they become VERY aggressive towards each other. Sassy usually initiates it and they end up biting, hissing and growling. Sassy stands over her and makes a strange clicking noise with his mouth.
It seems to be happening more often since we spayed Kat.
Are they playing? should I be concerned about their behavior?. They are so cute when they snuggle!.
Thank you for your time, I appreciate it!.
Pictures:
Happy times:
http://img.photobucket.com/albums/v308/amber581/SASSY/b835f5a8.jpg
Bad times:
http://img.photobucket.com/albums/v308/amber581/SASSY/1f7a559f.jpg

Answer
Amber,

Thanks for the pictures!!!!!  Yes, they are very cute snuggling together.  

On to the question.  The clicking of the jaws is standard hunting behavior.  I do not usually hear about it in relation with other cats, but you see it when the cats are looking at birds out the window or hunting a fly in the house.

Cats will play with each other incredibly roughly.  If you saw tiny kittens playing, you would be appalled at how roughly they play.  Cats have very, very tough skin (watch the vet have to punch a shot to get through the skin).  However, rarely will two cats hurt each other.  They are either playing or having a sibling spat.  In either case, the best thing to do is to ignore it.

Now that Kat is spayed and does not have her hormones driving her so much, she is less aggressive and Sassy is taking advantage of it.  They will find their own way through this and the best things to do is not to interfere.

All sounds pretty normal to me.

Best regards... Norm.