You cannot mine bitcoin with a regular PC - the only profitable option now is to use an ASIC like the Antminer S9. You need the raw hash rate that an ASIC provides.

>> Are GPU rigs capable of bitcoin mining or not?

So, no, GPUs are not capable of mining bitcoin. You can use them for mining altcoins.

>> But what is the resale value of an ASIC?

The value of an ASIC goes down as the bitcoin mining difficulty goes up and eventually, the ASIC miner becomes worthless. Chances are that you'll use your miner until it becomes worthless. Check this mining profitability calculator https://jblevins.org/btcmpc/ - it takes into consideration the change in difficulty over time so you can see how your profitability declines as the difficulty increases.