If you want the ultimate in speed, then nothing but a 15,000rpm sas/scsi drive will do for you. Im pretty sure that a raptor will be YOUR best bet though. The raptor is the fastest desktop drive out there, so yes buy that for the system drive.
As for storage, any brand is ok, so long as its not maxtor (avoid them like the death). Maxtor drives tend to be rubbish and break all the time. You best bet is probably hitachi, seagate or samsung. Just make sure the drive has "perpendicular recording technology" as it will be faster if it has this.
youre correct about the newer 7200s being pretty close to the raptors as far as sustained transfer rates, especially the maximum transfer rate (the raptors tend to pull ahead noticably for the average and minimum transfer rates though)
for an OS hdd, if cost and capacity arent issues, a raptor is the most ideal choice to make (fast 8ms access times help with locating system files, web browsing, boot times, system responsiveness, etc)
for transferring large files between hdds like youre wanting, going with raid 0 for both hdd storage arrays may be the most ideal setup (i dont recommend raid 0 for much, because there are many times when its just not worth the cost and hassle, for such little to no performance boost, and the increased potential of lost data and a corrupt raid array due to something as simple as overclocking your cpu, but this is one situation where it seems like it should make a noticable difference, as far as performance goes)... if you do choose to go with raid 0 (and even if you dont), be sure to make regular backups of your important data
if you dont choose to go with raid 0... just choose a few current large capacity 7200s, the performance will all be pretty similar between them though, even if theyre different models, within a few MB/s either way