With memory sooooooooooooooo cheap,get some more memory! I have 6 gb of memory and Vista and Windows 7 run FAST! My memory cost less than 50.00. Do a google search for your chips and the price may surprise you. I have no idea what anything costs in Argentina so my answer may be moot. If you can afford more memory get it!
RAM is cheap, agreed. If you can run Vista on it chances are you can run 7. I have installed a x64 copy on a laptop with only 514MB, but 256 might be a stretch. Im sure there will be people that would like to know the results, so if you do try, please post back the results.
You can also try sticking in a USB drive and enable readyboost, should help out a lot.
I have an old computer that I am running Windows 2000 on that doesn't run it too well with 256 memory. It uses PC133 memory, but according to Memory Giant, the maximum memory for this particular motherboard is two 128 memory sticks (two slots). Memory price is not the issue here. I haven't used this computer for some time now. Maybe I will attempt to install W7 just for kicks and report back here. Windows 7 on a Celeron processor and 256 MB memory?????? Well, at least the original 10 GB harddrive has been upgraded with an 80 GB.
To increase memory, I would have to replace motherboard, processor, processor fan, memory, power supply (120 Watt), and case because an adequate power supply would not fit this case, couple of case fans, and then my recently installed second hand 80 GB harddrive would want to be replaced with at least twice that. At least I could salvage my 3.5 floppy drive and the recently upgraded DVD-RW - but the beige would not match the gray or black case that I would have to buy............... NAH, I don't want a memory upgrade on that box.
I attempted to install Windows 7 build 7260 on a 12 to 15 year old computer with a Celeron processor and 256 MB ram. This computer has been upgraded to an 80 GB harddrive (from a 10 MB) and a recently installed DVD-RW drive. The computer proved unable to even read the Windows 7 install DVD. (this same disc worked on another computer, so it is known to be a good DVD). I just placed the DVD in the drive and rebooted and even with a functioning Windows 2000 installation, the computer would not even recognize the DVD. (Would not get past the "Boot from CD/DVD prompt". Pressing the "anykey" proved fruitless.) The DVD drive does function normally with other DVDs. After about three tries I gave up. That's MY experience at attempting to install Windows 7 on below spec. equipment. Others may have a more satisfactory experience, but that's mine.