Help me choose! (DDR2 on a Dell)

Hi to all of you,

First of all I apologize if there are several mistakes in my post, English isn't my first language...

Ok so I want to upgrade my ram (I currently have 2 dimms of 256mb each, DDR2-400) and I was wondering how to get the best performance without spending too much money...

I have a Dell Dimension 4700, about 1 year old, which has 2 empty slots left.

There a 4 principal options that seem to be possible for me:

1 - adding 2 dimms of 256mb each, DDR2-400, for a total of 1gb;

2 - adding 1 dimm of 512mb, DDR2-400, for a total of 1gb;

3 - adding 2 dimms of 512mb each, DDR2-400, for a total of 1.5gb;

4 - adding 1 dimm of 1gb, DDR2-400, for a total of 1.5gb.

Please tell me which is the best option between those four, regardless of total memory (1gb would be ok for me).

Thanks for your help!

P.S.: If you think that another option would be better, please tell me! (like replacing my 2x256mb DDR2-400 by 2x512mb DDR2-533, maybe?)