Were I in a similar situation, I'd probably just get the heaviest bike lock I could find, and run it through the hole on the monitor and one of the handles on the computer, then bolt it to something extremely solid (or run it behind a pipe, better yet).
That doesn't lock the computer shut, but I seriously doubt the average thief is going to be goofing around with opening up the computer to steal the drives--they'd be more interested in just walking out with it. The only reason to use a "proper" lock is to keep the computer from being opened, but that's not really necessary in this situation--that's more for offices or universities where people might well try to yank a drive or graphics card if they could get into it.
If you really want, get a seperate lock to put on the case itself, to keep it closed.
The idea of a big, heavy piece of metal holding the case down is to delay the thief by the few seconds it'll take to pull the bolt out of the wall. And for those saying it's useless, the average thief is probably going to pick the lock to get in, but isn't necessarily carrying a heavy-duty pair of wire cutters or a hacksaw--they're breaking into an apartment, not stealing bikes. That's not to say that they might not have one handy, but if they don't have one on hand it's something slow and noisy to mess with when there's easier stuff to steal--TVs or whatever.
It also depends on how easy it is to walk away from your place with stuff--if it's the sort of area/apartment where somebody could unload everything in the room into a truck before anybody would raise an eyebrow, then you're screwed. If, on the other hand, neighbors might notice them carrying stuff out so they're mainly going to be looking for jewelry or cash, then messing with a heavy wire/chain and potentially making a bunch of noise might not be worth it.
Hell, if you want to be REALLY paranoid, run a wire along the security cable that is attached to an alarm somewhere else, so that if the wire is broken the alarm goes off. Depending on the neighborhood that might do squat good, but it's worth a shot.