Windows Vista Starter was only offered in emerging markets. I haven't seen it on MSDN and therefore haven't had a chance to test it out. I do know that this version was limited in functionality to some degree, but I'm not familiar with its hardware requirements.

With your current hardware I don't think there are many software tweaks in Vista that will significantly improve performance. At least make sure the HDD is defragged, delete unnecessary temp files, turn off any unnecessary video effects in Vista. There may be some minor improvements possible playing with Windows swap file, but Vista's management of that has been pretty good in my experience.