I remember it being around $300 for the 100GB 7200RPM upgrade, but then if you factor in your 40% coupon (not that they exist anymore to my knowledge) it becomes a much more palatable $180. I had a hard limit of $1800 for my system though including warranty, but otherwise it would have been a good value.
5400RPM drives aren't bad though, I still load BF2, CS:S and DoD:S maps before just about everyone else. And for storage, I just got a 400GB drive and put it in an enclosure, worked great (until the enclosure died, still need to get around to replacing it).