I would not advise using Vista unless you put a lot of guts into a machine. That is borderline minimum as far as "acceptable" performance when using Vista. Heck, my P4 HT 3.06 lags somewhat with Vista Home Premium. I do not advise using Vista. My vote is obviously XP. XP will outperform Vista on any machine.
I will definatly have to say XP . To sufficiently run vista without any sort of lags you need a high power dual core , 2 gigs of ram and a decent graphics card .
Thats the minimum i would run vista on anyway
I am moreso saying XP, cuz with Vista you will have too much lag with those specs on your rig. Especially if you are playing games. Just web surfing, you could probably get away with Vista, but your FPS (frames per second) will probably plummet with Vista.
I feel he should run everything ok with what he has. As long as he has a gig of RAM which he has already. Processor wise, he's good to go. I'm using a fairly dated PC myself and it's running really good.