Very true, but I see you've also got the space for it. I suppose you're using 2 IBM travelstar 48gig HD's? AFAIK this is the most you can squeeze onto your empeg (for the moment)
I'l also thinking of adding such a drive to my empeg, so I know they're costly.

I've got a 10gig now, and I suppose the Empeg guys also use IBM drives? Would it be wise to add such a 48gig drive as slave to the 10 gig drive, or should I rather have them switch places and make the 48 gig the master and the 10 gig the slave? I know that in theory this should come down to the same thing, but there may be practical issues that I'm not aware of.

I guess you can say that there may come a time that I want to upgrade again and then it would be less work to remove the 10 gig and add a new drive. The 48gig also spins faster, but I guess this matters extremely little because I've read that the drives are most of the time spun down.