Hi Chrissie,
As Eric says, I see no reason at all that the SKY and Freeview boxes wouldn't co-exist, and record alongside one another.
I have no experience of freeview HD boxes as I built my own (computer-based) system. I often seem to hear good things said about Humax freeview HD boxes, but have no personal experience.
I'd not entirely agree with Eric's pessimism regarding the advantages. Even though we view our (40") TV from about 12 feet away the difference is very noticeable - so much so that I've become a bit of an 'HD snob', and tend to turn my nose up at anything not broadcast in HD. There would be even more benefit with a bigger TV (or with a smaller TV, sitting closer), but I'm unlikely to get purchase approval for that any time soon.
The downsides/disappointments are:
1) That the HD channels all have station logos, which I detest. But that may not bother everybody (if you have Sky, you may already be used to it).
2) Much of what's currently broadcast on the HD channels isn't HD at all, it's just upscaled SD and you'll see very little difference from a pure SD broadcast. ITV tend to be more honest about it, specifying 'Upscaled' in the program guide, whereas BBC like to keep you guessing. Feature films and major dramas/documentaries are usually true HD.