If you're not going to OC, the 975 is a much better bet. It provides you with 3.6GHz, wheras the 955 is 3.2GHz. It's much more future proof, and will help due to the 4-threaded CPU the Phenom II X4 is.
What I'm not quite sure of is if it's worth paying that amount for the Phenom II X4 CPU, which is still an old design. For THE SAME money (literally, even a bit cheaper) you can get a Phenom II X6 1090T CPU, which is a 3.2GHz hexa-core processor, fast enough for modern games with enough threads to last for many years to come in both gaming and the high processing market. By far a best bet. It would fit on an AM2/AM2+ and AM3 socket motherboard, so there is no need for it to NOT be purchased. Much better bet IMHO.