You will be happier with a folder for each torrent. NOT finished vs unfinished. These may well be sub-folders under broader categories - e.g 911, p***, SUSE, pirated MS, or how ever else you want to organize them.
Even with a couple of G allocated to caching (and this is where BC leaves the other in the dust), you can get up near 20 real I/Os to a disk drive and that may limit you. Using random other drives (but NOT primary / secondary sharing the same PATA cable) can help a lot. Should be using SATA these days, anyway, and eSATA to an external case that takes a SATA II drive and uses the (its different!!!) eSATA cable to your PC but that also has a cruddy old slow USB 2.0 for emergencies is very nice and flexible.
Use the new BC 0.71 "MOVE" feature carefully. It seems to have messed me up wrt folders somehow when moving between disks, but may be my learning curve issues, or files names that included a subfolder specification.
Then go BACK to 0.70 to stay sane, as the only 'goodness' in 0.71 is the MOVE function.