The best upgrade I have ever made with my system was when I invested in good quality interconnects and speaker wire about 4 years ago. The improvement was stunning. People tell me that I am nuts to spend extra money on a pair of cables, but I think THEY are nuts not to.
I use PBJ's for audio on my DVD, CD and LD. I took a BOX full of cables home one weekend and listened to all of them, and there were noticable differences in all. The PBJ's just seemed to be the most revealing of the lot and very nuetral. I also use Tributary's for S and digital.
Don't think you need to limit yourself to a dollar amount vs. the price of your equipment. Find a dealer that will allow you to try several cables, take them and spend a weekend with them, then decide. The cables that sounded best in MY system ended up not being the most expensive. BTW, Monster seemed to muddy my system a bit, a lot of that has to do with my gear.