I'm sorry to say that School Rumble has, by volume 4, developed a problem. It's trying to be serious. And it's starting to set up elaborate misunderstandings based on the fact that nobody can actually speak to anybody else in a meaningful way. And even the side stories about Tenma's sister are suffering.
I'd still recommend v. 1 & 2... and maybe 3. But if you're looking for laughs, never mind v.4
School Rumble vol. 3 and vol. 4 are both available from Right Stuf, Intl., an online retailer specializing in anime and manga.