any higher and you're going too far
Not really. The old saying is "you can never have too much RAM", and it's true. There's no performance decrease if you use greater amounts of RAM; in fact, you'll probably see more stable performance as it's less likely your system will every fall-back to using the page file which is slow and horrible. Also, more and more games and 3D applications these days are making use of greater amounts of RAM.
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Just remember that RAM is "paired". You can't put a random 2GB stick and a 4GB stick in the same paired slots together; check your motherboard to ensure it has 4 slots first, and then make sure that the 4GB RAM (either 2x 2GB or 1x 4GB) goes into Slots 2/4 with the previous 2GB of RAM (either 2x 1GB or 1x 2GB) goes into Slots 1/3.
Also, double-check the speed of your 1GB RAM stick(s). The motherboard will always operate all RAM sticks at the lowest compatible speed, so if you have a pair at 800MHz and a pair at 2300MHz, all RAM sticks will run at 800MHz, and you'll have wasted your money buying faster RAM.