Depends what you want to use it for......If you are planning overclocking then go for the RAM with the best timings( and reviews of course) and screw the cost, but, on the other hand, if you are not overclocking and just want a stable system then go for the value RAM at the corect speed for your Mobo and CPU...
Make sure that you get the correct RAM i.e DDR1 or DDR2....and make sure you get the right speed i.e as one example DDR400(if it's DDR1) (also called PC3200) or DDR533 or 667 if it is DDR2
There are other speeds of DDR1 and 2 available!
Also when posting tech questions about upgrading please list the components that you already have to allow us to get a better idea of what you need!
Hope this helps