added memory but computer does not recognise

i added a gig of ram but the computer does not recognise it...
do i have to do anything once i install the ram?
i have an intel imac 20"....Please any help would be great

No the computer should recognize the ram. Did you try rebooting?

well...not since i turned the computer on after i installed the ram. but shouldnt it have already recognise it

ok i rebooted but it still says that i have 512 mb of ram...

If it's not seeing it at all then it most likely isn't seated properly. Make sure that the contacts are properly made on both sides of the DIMMs and try again.

What memory did you purchase for your Intel iMac? Where did you buy it? What are the Specs? Is it installed correctly and seated all the way down lke Cazabam suggested?

http://www.welovemacs.com/mxma346ga.html
thats where i got it from...it has all the specs on the webpage

actually the memory works but only in one slot...if i try to put it in the second slot, a light blinks on the bottom right hand side of the computer and i get a black screen. for now i just have 1 gig in the first slot.

Put the 1Gb RAM in the slot the stock memory was in, and put the stock memory in the other slot. I had the same issue when I put the RAM in wife's iMac (same as yours) and that solved the problem. If that doesn't work, then the machine itself likely has an issue.

Baggss is correct. Also the RAM you bought is being sold for your machine and should work fine. The only other thing I can think of besides what Baggss said is if it still does not work after trying what he suggested, it also could be a defective stick of RAM or the machine.