I own a tracker 175, and it came stock with a Lowrance 37x fishfinder.  But, on my last trip out , I noticed that the screen is starting to blur.  So Im in the market for a new unit, and have been looking at the Lowrance 510 and 525 models.  Does any one have any input on either one of theses units?  Also, Im open to  any suggestions on other units for around 600 dollars.

You can't go wrong with any lowrance unit near that price range.  I have heard good things about most of their upper models.  I had a 332 gps combo for years and it worked great.  I also have a new HDS 5x non gps that works amazing.  Both of those you mentioned I believe would work great... and I think Cabelas possibly has a sale going on for the 510 right now.

For your $600 you can pick up a good gps/sonar combo.  You won't be sorry for the gps after learning to use it.  I hate to go on the water in a boat that doesn't have one.