Go to http://crucial.com and use one of their tools to figure out what you can put in and which configurations your system supports. You might want to buy the RAM from them too - good RAM, good prices, good support.
'Nother point to keep in mind--> Even when two sticks of RAM appear to be the same type, they may differ in density. I recently added a stick to my 5-year old computer and I had to get an older one because the newer ones were made with higher density chips. A low density stick will work in a high density slot, but not vice versa. I just googled "high density" "low density" RAM and got a bunch of differing results on how to tell which is which and whether you need one or the other, so you may need to do some investigating to verify what is compatible with your computer. Good luck.
Also to clarify, are the USB ports built in to the motherboard or an add on card? If you plug the device in, then start the PC , does it go? Is it a brand name PC or whitebox? Is it likely to be under warranty?