2 empty ram slots on my mobo, however one of them is close to my cpu fan and the heatsink on the ram would likely get in the way. Would I be able to mix 2 sticks of the new ram with 1 stick of the old, or should I just take the heatsink off of one of the sticks so it fits?
Nope, that combo will not run It's doubtful that 1.65v RAM will work together much less 1.50v RAM; the tollerance is ±0.1~±0.2v so your 1.80-0.15 = 1.65v vs 1.50+0.15 = 1.65v but that's the EXTREME. One is under-volted and the other over-volted and in my experience won't work or be reliable. They may work at 'SPD' of DDR3 1333 MHz 'assuming' there's a JEDEC #3 with 667 MHz see below. Use CPU-z to 'see' if there's a 667MHz @ 1.50v THEN the CAS timings on the CMZ8GX3M2A1600C9B MUST match.
Bottom-Line, it's nothing that 'I' would risk running in my rigs.
DDR3 1600 9-9-9-24 @ 1.80v
DDR3 1600 9-9-9-24 @ 1.50v
DDR3 1600 9-9-9-24 **** too much voltage disparity ****
The bottom has fallen out of DDR3 1600 2x4GB pricing so I wouldn't loose any sleep over it, there's a crazy deal every day on Newegg; I'm on that site like crazy. Tomorrow it'll be the same or better on G.SKILL, Corsair or Mushkin - Mushkin is due for a 'Shell Shocker'.