As far as which CPU to choose:
The K version offers two benefits. First it has HD3000 graphics over the HD2000 version found on the regular 2500. Since you have a 6970, you have no use for the built in graphics. So the regular version of the 2500 would be fine.
Second, it also has an unlocked clock multiplier. This is useful if you want to overclock.
So you have to decide if you want to overclock or not. If you do go with the K version. If you do not plan to overclock get the regular version.
Also, keep in mind if you do want to overclock the CPU you will need a motherboard with the P67 chipset. The H67 chipset does not work with the unlocked multiplier.