By default when you fit an add on video card the MBs integrated graphics chip is disabled, this feature can be turned off in the BIOS however all that this means is that you would have to use the MBs onboard video chip to get a display.
Please note that both will not run concurrently, it is either the add on card can be used or the onboard video chip and not both.
Your MB only has one pci-e x16 slot and one pci-e x1 slot which means that you would need a pci-e x1 HD 6450 add on card to pair with the present card that you have.
Yes your present card is crossfire capable but please note that this would require a quality brand PSU with a minimum output of 500W + any additional supplemental power connections that the cards would require, please see http://www.amd.com/u...overview.aspx#3
Crossfire or in the case of Nvidea cards Sli mode is mainly for power users such 3D designers, video renderers, games programmers and the likes, the average user/gamer can in the main get the required performance with a single but good quality and specification add on video card, be aware though that you also need to have other hardware to support and compliment the card such as a decent processor, sufficient Ram and of course a suitable PSU, if these don`t match up the system will bottleneck as the weaker components struggle to keep up with the faster hardware.
In short no crossfire is not always important and my advice would be to upgrade the present video card + PSU and depending on whether you are using a 32-bit operating system or the 64-bit version max out the Ram to 4GB for 32-bit or 8GB for 64-bit, that is if this has not already been done of course.
I hope that you find the above helpful and if you have any further questions or would like help choosing potential upgrades please feel free to give us a shout.