Well, regretfully I do not know of such a website. However I do have advice for you.
Read a lot!! seriously, start with a motherboard, then look at all the specifications: CPU socket type, fsb (front side bus) memory type, northbridge chipset, video interface (PCIex16 2.0, AGPx8 etc....) and everything you can find. Then for every word or acronyms you don't know or understand. look it up, learn everything you can about it.
Example => motherboard supports 16 Gb 240-Pin DDR2 SDRAM DDR2 1066 (PC2 8500) Dual Channel
look up DDR (learn what it is, different types i.e. DDR DDR2 DDR3 and what the number means (it's data transfers / second)
It will take a while but in the long run you won't regret knowing what you are doing when you are inside your computer or when something goes wrong.
And honestly, it is really just matching numbers and interfaces between your motherboard and everything else. Keep asking questions here if you really don't understand something and there are plenty of people who would gladly explain things to you.
good luck