Assuming stock speeds both the PII X4 955 and the A8-5600k should perform the basically same in games. Overall, the A8 is a slightly weaker CPU, but the higher clock speed basically makes up the difference.
The integrated graphic core of the A8 will do hybrid crossfire (or dual graphics) with any Radeon HD 6xxx series card up to the HD 6670 anything faster and the integrated graphics will be switched off. There is a Radeon HD 7670, but it is only sold to system builders like Dell and HP and it is really a HD 6670 with a new name. Therefore, installing anything faster than the HD 7670 will also switch off the integrated graphics. Not sure if hybrid crossfire will be compatible with any Radeon HD 8xxx graphics card. Lastly, installing any nVidia card will also switch it off.
So if you plan on installing a Radeon HD 6750 / HD 7750 card or faster card, or any nVidia card, then just buy the cheaper of the two CPUs.