What do you do with your computer? If you render in 3ds max, or use some other quad threaded software than yes it'll make a big difference. If your talking about just games, than no, you'll probably see a decrease in frame rates in a number of titles. Phenom is faster clock for clock than the old Athlons are, and some games do use more than 2 cores, but in most cases it won't make up for that great of a difference in clock speed.
Besides why get a Phenom I? The Athlon II X4 is in the same price range, has higher clocks, much lower power consumption/heat output, and superior overclocking capabilities.