[Howling]

As you mentioned, mass pvp is mostly CPU restricted. The Tabula Rasa CP raid dropping to 30fps is due to the cpu, not the gpu. Mine drops from 80ish to 20ish in HUGE raids on an 8800GTS 1680x1050 max settings, though that has lowered a fair bit since they changed the AA, with full AA I get about 60fps and drop to 15-18 in big raids. The drop is mostly caused by the CPU, mines a Q6600 @ 3.2GHz... if TR utilized multi-threading it wouldn't be a problem - but as with all games at the minute higher clock speed > more cores.
The HDD thing is all about cutting load times, it won't "boost fps" per se - it'll just cut loading time, ie if you're in an area and player x comes into view, your fps will spike down due to the delay in finding the textures etc needed to form the new player.. Now in big towns multiply this spike by several hundred different players and you find that your fps spikes very low for quite a long time.. but once you have all the players fully in view and they're loaded your fps will return back to normal.

imho the best card out at the minute is the 4870 due to it's cheapness and performance, that's my recommendation to you. Though the 4850 would be ample for Aion, it'll be the CPU and HDD giving you frame rate issues above anything else.

Edit: Oh and I don't recommend CF or SLi ever to be honest. Buying a more powerful single card will always get my vote, if nothing else than to remove any potential CF/SLi compatibility issues - which there are quite alot of.