Leaving any AV on right now shouldn't impact gaming. Quad core CPUs with 4-8GBs of ram is the norm now. I could see issues way back when with single core CPUs and 256-512MBs but not now.
Running an active scan might cause issues. We have enough CPU and ram power to not have a scan impact performance, but hdds are a different matter. I can easily see a single 5400-7200RPM drive have issues loading a game with a full scan in progress. But just leaving even a bloated AV running like Norton or McAfee shouldn't be an issue.