So choosing 2gb modules "for future" is just too much logic
yes -- this is an excellent point, and i agree. this is probably the last memory upgrade you are going to do, so buy according to that -- don't decide what to buy based on the idea that you are going to keep upgrading every couple of years, because it is far wiser to plan to buy a new computer in a few years.
you should still upgrade to at least 4gb, because that will make a big difference in performance at a low cost, even if you use the machine as a backup secondary computer.
but bamse's point is good -- don't make a decision about what RAM sticks to buy based on some imagined future upgrade that is not going to happen. all of us fall victim to this kind of thinking over and over again -- we spend more money on things because we say to ourselves "well i will buy X because one day i might want to do Y", when the reality is that Y is so unlikely or far into the future that by then we will want to buy something different.
i come from a long family of people who exhibit this behavior, and i have identified part of the reason why it always fails. it happens a lot with power tools -- where one says "i will buy this tool because ONE DAY i might want to build table legs".. but the critical flaw is that in the very very rare chance that iever really want to build table legs, on that day i will inevitably start searching on the internet for a BETTER table leg creation tool. in other words, if the day ever comes when i need the fantasy tool that i am buying ahead of time, i will inevitably feel like i need a better tool for the job.
short summary: don't try to plan too far in the future in terms of computer upgrades -- in the non-near term future you will want (and be better off with) a new computer.