I have XP on my work machine, Vista 64 on mt workstation and Vista 32 on my home laptop. With the 4GB of ram you can make use of all 4GB with Vista 64 but no major loss if you only run Vista 32.
Depending on your Windows abilities, I would say XP is the simpliest of the two and generally less memory intensive, Vista isn't the worst thing in the world unlike what you may otherwise here but the nagging prompts asking you to approve changes do get old.
If everything you intend to install works with XP then stick with that. At some point you would probably consider Windows 7, improved memory usage and smaller potential footprint might be desirable.
Either way, I suggest you download the latest service pack for which ever OS you decide to go with and install that ASAP after the install so it will reduce the number of patches you need to download/install. I think on a fresh XP install with no SPs you will probably go through 4 or 5 reboots.