Does having unequal size RAM affect performance? I want to make this upgrade because I run out of RAM while playing games and doing other resource-intensive stuff. My PC is a Dell 7577. Will it run in single channel or dual channel after the upgrade?
After the upgrade it may run in single channel, fully or partly.
For full dual channel, both RAM sticks must be totally identical in all respects. Otherwise, with 4GB+8GB, and if your motherboard supports it, you will be running in Flex Mode, where only the lower 4GB of both sticks run in dual channel mode.
However, dual channel is unlikely to have any impact on the performance, so isn't an important consideration here.
Yes you can.
See the specs:
Per slot: 4 GB, 8 GB, 16 GB
Total memory: 4 GB, 8 GB, 12 GB, 16 GB en 32 GB
You can only get to 12GB with one 4GB and one 8GB module. But they don't say 20GB or 24GB.
Most motherboards will drop to single channel if the channels have unequal memory sizes. While this is a significant effect on synthetic benchmarks, it's a very minimal effect in the real world. In any event, the benefits of having more RAM (such as additional caching) will drastically exceed the penalty of losing dual-channel when looking at overall system performance.