@Joker08: Always a good question - always hard to answer! As @Kurtiskain mentioned, it mostly depends on how you use your computer. Unless you are a 'power' user of some sort, you probably would be better off with 'more' than 'better.' And ALWAYS better off with quality RAM from a quality manufacturer.
Be advised that if your OS is 32 bit, it will not use more than ≈3 - 3.5 GB of RAM if you go with 4 GB.
Also, for the 'normal' user, typically XP can perform very well with 1 - 2 GB and Vista needs all it can get - but, at least 2 GB.
Best of luck with your upgrade!