FWIW, I had a similar configuration to yours (just an older MBP) and installing an SSD helped immensely. I can hit 200% CPU load and not even realize it until the fans kick in…
and as soon as i booted Windows XP in VMware, well, took me a while to be able to reply to this post (after the vm settled).
I also that you might be saying "d'oh" but i've had Gentoo running on this metal and a "similar" environment AND compiling stuff with -j4 doesn't freeze away my UI.
My user experience with "OSX" is that it's, way more prone to unresponsiveness due to load, but hey, who cares :P clicks Time Machine
Not that Anonymous Guy On HN is worth much, but get an SSD - it'll be the best upgrade you've ever purchased.
(Or maybe I just needed to sidegrade to Linux. :)
You can also get a brackets that replaces your optical drive, and allows you to fit a 2.5" HDD. I have one in my 17" non-unibody MBP, and it's really the best of both worlds - I keep OS X, my working files and apps on the SSD, along with my main Windows XP web testing VM. My iTunes library, media, and games stay on the HDD, along with a Boot Camped copy of Windows 7 (though it's a pain to get the installer to run without an internal optical drive). I keep a cheap Samsung bus-powered DVD burner in my bag, but in reality I rarely need it. I think OWC sells a bracket for Macs, but if you can figure out exactly which bracket you need, a site called newmodeus.com sells them for almost every laptop ever made for considerably less.
I really do believe my SSD is the best upgrade I've ever spent money on; no computer I use from now on will be without one. It's not so much that the computer is faster; it's more the feeling that the computer does not grind to a halt or slow down, no matter what's going on. (I may have compared my computer to the Terminator amongst friends once or twice… it just doesn't slow down.)
SSDs do a lot to reduce loadtimes, and thus make your computer seem much faster, but they do little for making your programs run full-speed-ahead constantly. Most every application out there blocks on network connections, user input, or just plain old throttles itself.
For that matter, I can max out my cores just using a couple dozen instances of mplayer, playing several movies at once off of a usb removable harddrive...
Virtual memory paging to/from disk. This is probably why the new MacBook Airs feel faster than the CPU+RAM specs suggest.
During standard home computer operation, both the CPU and the disk are generally quite idle.
Even with a perfect scheduler you're going to have to wait on I/O. Disk speeds are the limiting factor on most machines, and this goes double for laptops. I highly recommend getting an SSD.
This isn't to say that there aren't real limits or that BeOS was perfect (far from it) but simply that there's considerable room for improvement before we start hitting theoretical limits.
(A really fast expensive SSD is around $400.)