I did purchase 2 Barracuda IV 40gigs after finding out the same as you did (40gig per platter). But, I want to get a boost in performance, too. So, I will be running the 2 40gigs in raid 0 (striping). This will definitely produce more heat but I already 2 WD drives in there and the system runs stable.
I sent in my 2 Barracuda IV drives over a week ago to Seagate to get the Raid 0 "friendly" ones. The RMA status says they got them -- I'm just waiting for the new ones.
Another option would have been to get the new single platter 60gig Barracuda Vs but those show slightly higher dbA even with only 1 platter. I want to minimize the noise as much as possible.