this HZ thing is monitor dependent, are you on TFT?
if u r, then changing it to even 120Hz wont bring difference, as TFT's run on 60Hz. but if u r on CRT, then go for highest Hz setting it is showing, as it will hide those which ur moniti cannot display. and higher the refresh rate setting in CRT, it will prevent flickering (higher means what ur monitor can handle at most)
else, changing to D3D or openGL wont do any bad to your pc, it will run fine. as you'll have problem in games, as if u r forcing OpenGL an the game is on D3D. so better is application preference.
as OpenGL is mainly used for 3D designing, therefore you'll see workstation cards like rage, quatro have more OpenGL capability than these radeon etc.
Intel Core2Quad 2.66Ghz 12MB L2-Cache
4096MB DDR Kingston Hyper-X
XFX 9600GT AlphaDog
Seagate 2 X 320GB SATA2
LG L246WH Flatron 24"