Also, you must consider the following:
- if you have a 32 bit OS, you'll only have 3.2Go of recognised RAM
- depending on your CPU, it might not be worth putting 8Go anyway: if you've got an old P4 or a small C2D (E2/4), they just don't have use of that amount of RAM
- finally, you might not need it: unless you have a wide array of progamms open at the same time, you won't see the difference between 4 or 8Go.
If you're using XP as an OS, I don't event thhink that over 2Go is usefull.
Think about it before spending any money on RAM.