Time Warner Cable is required by law to provide you CableCARD(s) (for a small monthly fee), to support your TiVo S3 or TiVo HD DVR. They are the most affordable options currently in production. There is also a new Moxi DVR that supports CableCARD, but it is a bit more expensive than TiVo.
Unfortunately, technology will keep changing, and personally, I would wait another year or two before buying another DVR of my own, until it is clear where tru2way is going and how quickly it will become a factor. So I would recommend you chalk this up to bad timing, and hold off, continuing your current services, at least that long.
If you had asked a couple of years ago, I'd think you could have purchased one of these CableCARD-equipped DVRs with a relatively high probability of the investment paying off in the long-run -- now, I just don't think that is the case since so much time has passed.