well, sounds like you've had good luck with the programs. that's a pretty decent sized board, and it sounds to be performing admirably. 15-30gb/mo isn't a ton, but if you're not sending a lot of graphics, that's a decent bit of traffic.
generally for high traffic i'd suggest vbb. but, i've seen it break with massive traffic as well (even literally brought to a halt - on dedicated).
php is a wonderful language, and you can do cool things with it. like i said, you just have to be careful with it. i had a friend with a phpnuke site that was getting about 60k uniques or so daily (have no idea on raws), and it broke CONSTANTLY. although _most_ of the breakage was with a plugin, but the core site had more than its share of viscous errors as well. it also occasionally did some really screwy things to apache, requiring the machine to be manually rebooted (apache or mysql would consume nearly all of the processor, making everything dog-slow).
also i'd give props to your hosting company, they're doing their job quite well if it's not being slow or holding up other clients. there are a lot of hosting companies out there as well that couldn't handle the users on static loads. i can't stand that, i'd switch in a heartbeat if any of my hosting providers did that
but so far i've been VERY pleased with my more important hosting. they cost a few bucks extra, but it's worth every penny.