I DM, and own the majority of the 3.5 books... and I expect I'll end up owning the majority of the 4E books as well.
Of the other folks I play with... the ones who DM regularly tend to have the majority of the books, or have ready access to em (since the majority have been college students, this includes anything the gaming club has, or things other members have).
Among the people who just play and don't DM, I'd say the vast majority have had their own dice. Maybe half have had their own PHBs, while a growing number over the last year used the SRD for character creation, leveling, and looking up PHB spells.
Very few have anything beyond the PHB... but those who do tended to have the majority of the 3.5 books. These were folks who, at least when I gamed with them, did not DM.
Only one person I've played with has a significant number of minis... he, however, has what I can only refer to as a metric crapton. And plenty of other mini-oriented stuff. Personally, I don't have anything for minis or battlemats - either I'm gaming with the guy who has everything, a group that doesn't tend to use minis, or we're playing online. I could see picking up a battlemat if I was in a group that wanted to use one, but I'm more likely to use coins, dice, and bottlecaps than invest in minis.