I'm in a similar situation, I have a q6600 (mine is overclocked) 8GB of ram and an SSD, and I am currently CPU bound. When i 5 box (all alts are set to lowest possible, and the main is set only slightly higher) and all 4 cores are working at or near to 100%. I am planning on upgrading to a core i7 930 with 12GB of ram. (I'd go 9GB if they made 3GB sticks that were affordable)

I disagree that SSD for wow is pointless. EVERYTHING loads faster. I recently logged in a buddys computer, and in dalaran it took forever for the other characters to show up, where on my system its near instant. SSDs are very affordable now, you just cant fit an OS, games and media files on em just yet without breaking the bank.

I used to run 2 computers, but the added complication of the extra mouse and keyboard (for other non multiboxed tasks) and lack of windowswapping with isboxer made it not worth it for me. Only thing I missed is that if my main computer crashed, my other characters on the other PC were still able to play, and I could communicate this to my teammates.