Stick to 4GB, also you can make use of the 2GB you already have.
Just be sure to get the pairing right on the motherboard. Usually it's AB AB.
For example if you currently have 2 sticks of 1 GB and buy one of 2 GB:
[1GB] [2GB] || [1GB] [empty]
Also helps if the bus speeds match.
There's no reason to scrap your current RAM, so if you get an awesome deal on a set of 2 sticks of 2GB there's no reason not to go for 6. All 6GB will do though is make it harder to notice a memory leak.