Well, I'm a 72 str fire/light bow farming with 9gap so maybe I can help you out a little. If you have another high lvl acc and 2 computers, or have one really nice friend/guildmate then pfarming is definately the way to go (insane sp/hour, I've heard people talk of 10k+ with tickets). Personally, I tried farming at 16 at den and only got about 25k before I had enough of watching myself die 50 times over and over. As for the higher level farming, I say go with either 64, 68, or 80. If you want to go 64 farm, Power Earth Ghosts are probably the best (atleast, here in Greece, there aren't that many people in the lower PEG rooms). If you want to go 68 farm then I'd say Earth Ghost Solder in the lower rooms. And finally, if you want to go for the high 7x-80 farm... well... anywhere you can find a decent spawn

. IMO, 72 farm sucks... I have decent garms, blues, and a 72+5 bow, but anything that I can effectively grind at either a) has a shitty spawn rate (not worth running all over) or b) is filled with bots. So yea, hopefully I helped a little.
EDIT: I can get around 2ksp/hour in the nachal room when there are only 1 or 2 other people there (gt and spt). If I could go back, I would prolly farm at 64 at PEGs and save up for a sox d8 bow, or if you can afford it by the time your 64. This is definately the best option imo (if there aren't many people in lower PEG rooms...you might want to check it out or ask someone else to see before you plan for it though)
EDIT2: Also, if you are anything like me and hate having a gap between 5-8 (trying to lvl and get sp... not as easy to see gains and just feels like an endless grind with no evident returns), then once you choose your farming lvl (lets say 64), when you get to 9 lvls below it (lets say 55) get all your bow skills maxxed to that lvl (lets say 55) and then level ONLY your mastery until your next lvl is desired farming lvl (64), then just cursed heart down the 8 mastery levels and bam, insta-9gap.