RAM compatibility

Is there a website where I can look up what kind of RAM my computer has, and what kind of RAM would I need to buy to ensure compatibility ?

I've always used Crucial as a way to quickly drill down by manufacturer and model, even if I don't buy it from them. Their Memory Advisor Tool will tell you the number of slots on your system, as well as what type of memory (speed, number of pins, size, etc.).

The next best option in my opinion is to look at your manufacturers support site to see if they list the information specific to your system (e.g. if its a Dell you can use your service tag to look up your specs, which would include information on memory compatability).

Several of the major direct to consumer RAM retailers such as Crucial and Kingston Technology, have RAM type search tools for major pre-built systems.

For pre-built systems the System's User Guide from the manufacturer or distributor's website will normally include this information. E.g. HP and Dell.

The User Guide for a given motherboard of a small-shop or home built PC will also detail what type of RAM you need or want. Some manufactures even offer lists of tested compatible or recommended models (Asus does for example).