GPU's are much more efficient than CPU at mining, but ensure that you calculate your profitability for your particular card based on its hashrate and your cost of power. The 7790 is a rather old card and only has 1GB of memory so hashrate may be low for the amount of power it consumes. If your power is free, then perhaps just throw whatever hardware you want at it and see what rates you can achieve. If its winter it can be a good way to warm the house :)

Also you do not need to run them in crossfire, they should be picked up individually by the mining software. Disabling crossfire can help reduce stability issues.