If you want to specify the amount to receive, go to the 'Merchant' page (after you selected in the 'Receive' page the @account that will be used to receive XMR).
Monero can be mined on both CPUs and GPUs, but the latter is much less efficient than the former. You can get an idea of how your hardware performs compared to others, using <a href="https://xmrig.com/benchmark">xmrig benchmarks page</a> (some results might be out of date).
