Generally TV Tuner is much better in MC11 than it was in MC10. I had constant trouble with my standard BrookTree circuit based card (analog). Now it works almost flawlessly. I have not used it for time sifting, so I really can't say how well that part works in MC if at all.
I guess you could get by with your current PC if don't set your recording resolution too high. However, it is going need a lot of HD space, because I don't think you will be able to use any advanced compression codecs on the fly.