In my experience, Xp is the best OS on a pentium 4, but Debian is always a good alternative, particularly given the systems age.
In all honesty, go on newegg and pickup a low end tower.
http://www.newegg.com/Product/Product.aspx?Item=N82E16883157303
194 bucks will get you to a core 2 duo, and more modern ram (ddr2)
http://www.newegg.com/Product/Product.aspx?Item=N82E16883108662
279 bucks will get you into a system that will run 7 like a champ, and can be upgraded a lot more.
In all honesty, go on newegg and pickup a low end tower.
http://www.newegg.com/Product/Product.aspx?Item=N82E16883157303
194 bucks will get you to a core 2 duo, and more modern ram (ddr2)
http://www.newegg.com/Product/Product.aspx?Item=N82E16883108662
279 bucks will get you into a system that will run 7 like a champ, and can be upgraded a lot more.