What would be the better computer for graphic arts?

Multitasking requires the most cores, so the Phenoms are what you want, you also may want to get a better video card, at least a 5770 or a GTS 450 represent the best balance of graphics performance and power consumption, 5570 is a low end gaming card and 5450 is an HTPC card, both cost sligthly less than the 5770.