It's just that RAM is usually an even number, for example 4Gb(2x2), 6Gb(3x2), 8Gb(2x4/4x2), and so on.
As for the CPU, I tend to buy intel, they've always worked for me in the past. However that would therefore require a new mobo. May I ask how much youre willing to spend, and whether or not you would consider a new mobo?
Edit- Plus if you were to get an AM3+ mobo, you would still be able to get a AMD CPU if you would prefer