Well that's a load of rubbish as your machine far exceeds the minimum specs. The fact it's also available with Vista installed makes a mockery of that claim.
It's possible that Windows 7 would have most of the needed drivers included anyway. If there's any that it doesn't cover, then the Vista drivers would probably suffice.
You have 4gb of ram so ideally you would want Win7 x64, but if Sony didn't provide 64bit drivers for Vista then you'll be stuck*, and running a 32bit version of Win7 would be you're only option.
* (If Win7 required drivers to be installed for certain hardware to work that is.)