hi all, why is it so difficult to get hold of some ram for a computer!

can someone explain the difference between these 2 please:

http://cgi.ebay.co.uk/ws/eBayISAPI.d...9375&rd=1&rd=1

http://shop.kingston.com/partsinfo.a...TH-XW4200AN/1G

I thought the ebay one would fit my comp but the bloody gap in the ram is 3 mm away from my original cards.

thanks