In the U.S., 8 GB (2 x 4 GB) of DDR3 1600, CL9 RAM is very inexpensive.
Here's a list of current offers (prices low to high) from Newegg starting at $35.99 with free shipping.
If you're in the U.S., or you have access to vendors with equivalent pricesyour best bet may be to buy a matched pair of sticks and sell or trade your single stick with a friend.
Be sure to check your motherboard manufacturer's site for their memory compatibility list. If it's an older board, the list may not be current, but you can check the various RAM manufacturers' sites for their models that are compatible with your motherboard, and you can search Google for your motherboard model number + various models of RAM that look intereresting.
Be VERY sure you know the voltages your motherboard can provide (1.50 v, 1.65 v, etc.). The info should be in your user's manual, or you can read the available options in the BIOS. Using lower voltage RAM uses less power and produces less heat.
Hope that helps. Good luck.
