I bought the wrong RAM

Last Thursday, I bought a new RAM for my computer to increase its current capacity because my current 1GB memory cannot handle some of my photo editing I'm doing since I photograph using the large format. With all the editing I'm doing, the computer is starting to "hang" already. However, when I tried to install what I bought, it won't fit the socket of a DDR2 RAM because apparently I bought a DDR800 RAM without checking if its was the correct one for my computer. I was so disappointed because it was such a waste of money. Maybe, I can sell it to someone because it's still brand new. I hope my friend can help me with this dilemma. Next time,.I'll try to checke and verify my purchases. (sigh)