As long as the tuner is just dumping the MPEG2 stream to the harddrive, it shouldn't effect performance at all. There shouldn't be much CPU involved. Even if there is conversion going on to something like h264, as long as the card does the compression in hardware instead of dumping it to the CPU through software, it still shouldn't cause skips to your recordings. Even if the capture card is entirely software, as long as it tags it's threads as higher priority your recording should be fine, with gaming taking a framerate hit if you happen to be playing a game at the same time. Even then, you've got four cores, most games really only want three as they were designed for the Xbox 360's three-core CPU, and usually one of the cores is taxed more than the others. This leaves a full CPU core plus partials on two others available for video duties.
It should be fine, just look in to how the Ceton card handles recording before you make the purchase. I'm not familiar with that card. You want something that just dumps the MPEG2 stream to storage, or transcodes on the fly in hardware from broadcast to h264 with minimal CPU use.