Yes its absolutely compatible and in fact a good choice.. If you are worried about mixing it with your existing RAM modules then give us the details of your current RAM and we'll let you know in case of any complications..
If they can mix without any problems that would be awesome, the more memory the better, if not, i'll live. I currently just have 2gb and I def feel like my system runs sluggish because of that sole reason.
Some motherboards are sensitive to mismatched ram. Even the same part number can have different characteristics. That is why ram is sold in kits. The key specs to match are DDR2, and 1.8v.
1) Check the Gskil ram configurator to verify that the 4gb kit is compatible with your motherboard. I suspect it is. You want assurance from the ram vendor that you are compatible if there should be some problem.
2) Go ahead and install the 4gb kit. You will, at least have 4gb which should be plenty if you are not multitasking.
3) Install the old 1gb parts, giving you 3gb on each channel. They seem to be compatible. Your ram will operate at the lowest common denominator of specs. That is not all bad, since the value of lots of ram will overcome any minor reductions in speed or timings. You do have a 64 bit os to take advantage of 6gb, right?
4) Test with memtest86+. You should get NO errors after a full pass or two.