So I am ordering a PC on CyberPowerPC and I have to choose a graphics card (There isn't an option for none) so I am getting the 6450, then buying the Sapphire 7950 separate since it's cheaper on NewEgg. My question is, is will the 6450 be useful to me at all? Can I do a CrossFire configuration with the 7950 or will I just have to get rid of the 6450?
You can not crossfire those two cards. They have to be from the same series, so like two 7950's or a 7950 and a 7970. Just keep the 6450 in case you ever need it again. Maybe you'll need it for troubleshooting purposes, or for a new computer.
One thing that concerns me though, have you changed the PSU since you ordered the PC? The reason is that the 7950 will take much much more power, and usually pre builts will come will cheap OEM PSU's that are really no good.