Your best bet is a 2 x 2GB kit if you want 4GB total. This allows you to take advantage of dual channel. For the RAM to run in dual channel you need 2 matched sticks. This will give you around a 5% performance advantage over a single stick.
It's best to buy RAM as a matched set. If for some reason you can't then just get 2 of the 2GB Corsair kits.
Your motherboard will support up to 8GB so you could get 2 of the 4GB kits but you will need a 64 bit operating system to use more than 4GB.
If you can only get one of the listed modules for some reason the get the single 4GB Corsair stick. You will lose the advantage of dual channel but it's not a huge loss.