If you are a big game player, then I'd assume you will be running Windows. The big performance killer on Windows is usually swapping memory from RAM to virtual memory on disk and back to RAM again. So the more RAM you have, the less this will happen. Disk I/O kills performance.
Also, some video cards will claim to have n amount of video RAM. But in reality half of that amount is being shared with the pc RAM. Mid to high end brand name video cards like ATI and nVidia won't do this. So just buy wisely.
Tom's Hardware is an awesome resource full of hardware benchmarks on many different custom systems using many of today's popular games.