More RAM is usually the better choice, so combining them would be my personal choice. But, not all DIMMS "play nicely" with each other... But I'd still at least try to install them both.
Regarding memory timings:
If you can, manually set the memory timings to those of the 533MHz DIMM within the BIOS. If you don't know the proper settings, download CPU-Z
, run it, and see what your system is currently set to (Memory tab), as well as what settings your DIMM supports (SPD tab) at different frequencies. If that seems a bit complicated, you should just be able to select "SPD" within the BIOS, which tells the system to default to the DIMM's pre-set timings.