I am not sure how much you will need really. I would get 2 litres just to be safe and at least you will have some spare for future top ups or if you need to drain it down again for some reason. I know mine takes 2.5 litres but it's not the normal setup as it has 12 feet of copper tubing, around 6-8 feet of tubing and two 60mm thick triple rads. Surely you are not going to cool just the cpu with that kit? It could handle a gpu as well.
I plan to cool only the CPU at first until I get a better graphics card for the new system.
I plan to use my Gigabyte GeForce GTX 580 OC card from my current system only just to get the system build and up and running then I will be saving up for a more powerful graphics card for the system.