You do not need 4GB+ of ram for Vista to function correctly, it will work just fine with 2GB. Also any dual core processor of atleast 2Ghz will run the OS perfectly.
Here's a little hint turn off the following:
system restore(system protection), volume shadowcopy(services), indexing(indexing options), readyboost(services), superfetch(services) and auto defrag(task scheduler)
Turning off those services and features will do wonders for Vista's resource, processor and hardrive usage.
I run Vista x64 and at idle my memory consumption sits at 1.2 GB, with processor usage at 1-3% and almost no hardrive activity. Those numbers are with about half a dozen third party programs running. If nothing but the OS services are running my ram usage falls to around 700MB at idle with the processor and hardrive usage remaining about the same.