System RAM is going to be your major issue - my i7-920 with 12GB RAM was capping out at 10 clients when I last MBd Eve... http://www.dual-boxing.com/threads/4...149#post382149
Neither the CPU nor GPU were really stressed, barely hitting 30-40%.
It's all about the RAM!
At a guess, with 8GB, 6 clients would ok, with 7 you might start paging.
Should be relatively cheap to upgrade from 8GB to 16GB RAM, though, especially if you have free slots now.

Can't comment on how to make your ISK these days - made my fortune mining ice, then they decided to nerf ice mining so I decided to stop giving CCP my money.
By the time I quit, 5-box null-sec ratting with Typhoons (cheap battleships, firing cruise missiles from 130km) made more money than 10-box ice mining!