Trans/clutch prob?
 

I just recently bought a 05 rx8 manual. It run excellent but I have one issue. the shifting are great but when sometime I put in reverse, it grinds. I don't know if it a small problem that can easily be fixed or does it need to be changed and become expensive. The reverse stills grabs but at time it doesn't. So is this a big issue that need to be fix right away or is it small?

If the other gears work fine then the clutch isn't likely the problem. Does the shifter feel like it goes all the way back when it grinds? If the shifter feels like it goes all the way into gear and it still grinds you probably need to open up the tranny to fix it. My last truck had a problem like this, I drove it without too much difficulty for about 6 months until I got it stuck and really roasted reverse.