I agree with the above. I would avoid buying RAM off eBay, you will often be left with little or no come back and RAM can be so tricky to get right.
I suggest mrmemory or crucial websites for RAM. Their prices are as good if not better than eBay and you get warranty even with the Value / No Name stuff. I have build hundreds of PC's using this value RAM and it works fine.
Make sure you get highest speed your computer can take, the crucial memory detector will find this out for you.
When you fit the RAM its important that you clean any dust from the RAM slots and also make sure your PC is completely powered off and earthed (use antic static mat or straps etc)