Maybe this will help:
http://support.dell.com/support/edocs/systems/dzuul/specs.htm
It appears from the specs that 512Mb ram is the maximum for your machine. I to do think that is enough to run Vista at least with any satisfaction. I think at least 1Gb and more would be better.
Sorry, hope this helps!
Glenn
I have a Dell Dimension 4100 that has been upgraded before so it now has 512MB of RAM and a 1.1 GHz Processor. I was told that to run Windows Vista Home Premium I would need to upgrade to at least 2GB of RAM. Is this true and would I be able to upgrade that much on this computer? Thanks

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic