Searching for MacBook White 2008 compatible RAM

As the title describes, I am searching to upgrade the RAM of my MacBook White 2008 (not unibody).
I want to have 4GB (2GB+2GB).
I have tried in the past 2 times to change it on my own, but the MacBook couldn't boot. I asked the guy at the shop, to be compatible with MacBook etc. but still, couldn't work.
After failing to put the new RAM into the laptop, I put back the old one, but still had some problems (macbook not booting) untill it finally worked. What should I do?
I really need more RAM because I m working with Photoshop and similar programs..

Any ideas?

Have a look at Crucial.com. Enter the exact details of your Mac (you don't give details above but as an example here's a link to the White, 2008, 2.16 Core2Duo Macbook)

Computer memory upgrades for Apple MacBook 2.16GHz Intel Core 2 Duo (13-inch White) MB062LL/A Laptop/Notebook from Crucial.com

Though if your experiencing boot issues anyway there could be other underlying issues