That depends. HD or regular, into what format, and what generation (age) drive? I'd imagine you could handle capping standard-def into mpeg2 or so, but say HD OTA might pose a problem.
Actually, the cache on a hard drive has very little impact on performance for streaming media type operations. The cache is there to either pre-fetch data that the drive is expecting you to use soon or keep around data that is used often. Streaming media benefits very little from cache because the same data is almost never read more than once and a 2MB (or even 16MB) prefetch is very small compared to the amount of data the drive is pumping out.
I don't happen to have benchmarks around to back this up, but theoretically, at least, getting a bigger cache would be nonproductive for what you want to do. (however, if it doesn't cost more, I'd say go for the bigger one in case you decide to use your drive for other applications as well)