Vostro 1500 ram problems

Ok so I just got my vostro in and being my first laptop I figured it had 4 ram slots like my desktop so I was going to add a 1gig stick of ram to compliment the two 512's it comes with. Well after taking off the keyboard I realize it only comes with two slots... so I think no biggie i'll just instal my 1gig stick now and order another 1 gig stick later. Now here comes the problem... I initially instal it underneath my keyboard since I already had it opened and it would make installing the second stick later easier. My laptop wouldn't come on... it just stayed black. So I put the original 512 stick back in, flip my laptop over and instal my new stick of ram in the other easily accessible slot on the bottom of the laptop, boot it up and it's working. Now i'm trying to figure out why it wouldn't boot up the first time when I installed the new stick into the slot under the keyboard... I plan on buying another stick of the exact same ram but I don't want it to get here and it not boot up again.
http://www.newegg.com/Product/Product.asp?Item=N82E16820231069 is the ram I have. Anyone have any answers for a notebook n00b?

 

On some laptops w/diff size RAM order is important, I had IBM and Dell like that.

 

It is probably configured that it uses the slot on the bottom first, like the first slot is located on the bottom, and the second slot is under the keyboard. It should be nothing to worry about.