So, I have finally decided to buy a X201s with this CPU
CPU: Intel® Core i7-640LM (4MB Cache - 1066Mhz FSB - 64-bit - 25watt)
Turbo Frequency Max 2.933 GHz

Should I go with the original 32 bit Windows 7 with 3 Giga RAM, or with the 64 bit version with 4 Giga RAM?
The seller let me choose between the two options.

But I don't know so much about the differences..

If that matters I have an HP 1020 printer and a NETGEAR DG834G Modem Router to be connected to the laptop.

A.

If you have more than 4GB RAM installed and want to take full advantage of it, definitely you need to install WIN7 64. Or you can install WIN7 32 with only 3GB RAM recognized, and you can install RamDisk to convert your unused RAM into a virtual disk, which can be used as the system temp folder and browser cache.

Installing 32- or 64-bit version WIN7 does not matter which printer and router your laptop will be connected to.

Definitely 64-bit. Will run fine with 4GB of RAM. I think under normal use ie no CAD/CAM stuff and infrequent use of Virtualization ie XP Mode etc. you are fine with the RAM. I see no reason to run the 32-bit version.

64 bit will run just fine...you can even consider 8GB of RAM since the price has dropped like a rock.