I hate the thought of throwing out old computers, but I don't think I'd bother upgrading an existing computer to Windows 7. I like 64-bit Windows 7, but I find that Ubuntu Linux works very well on older computers. I'd be afraid that Windows 7 would slow down most older computers.
Of course, the prices of new computers may be much higher where you live. I can get a fairly good Lenovo Win 7 computer (4 GB RAM, 1 TB HD) here in California for about $425 US.
-Joe-