Windows 7 will run fine, but i'd still grab a gpu for the aero effects.
If you dont want to buy a gpu, you actually could use windows xp or lubuntu linux.
The latest version of firefox for xp runs mostly all websites. You can run the latest version of apache open office on windows xp and type documents fine. Sure, you wont be secure, but if you stay away from shady sights, you will be fine.
I actually love windows xp and most of its flaws are caused by older hardware or slowly declininv software support. Used xp 32 bit on an older laptop with an I5, 12gb ram, intel hd, and a 240gb ssd. Latest version of chrome can run toms hardware and youtube at 1080p. and there are plenty of fun xp programs. Heck NFS se2 runs amazing with nglide.