For those who already own the original Serious Sam I highly recommend to not buy HD, it's exactly the same thing except it has better graphics and has better multi-player support (trying to play the first one on-line won't be worth it considering how rarely people actually play that older version on-line, when they can, that is), unless of course you're a graphics whore then, heh, go for it, and that at price I guess it wouldn't kill anyone's ego.
And indeed Second Encounter HD might well be released sometime this summer I would presume, or perhaps before who knows, but it will be released in HD, it was confirmed already. The thing is that once both are released in HD they will do the inevitable Serious Sam HD + Second Encounter HD pack for a special price, either on Steam or not, but it will happen for sure, something along the lines of «pre-order Second Encounter HD now and get Serious Sam HD for free!» or «Buy SE HD this week-end and save 50% on SS HD!»...
In the end, if you really want to get Serious Sam HD although you've played the original then I suggest to just wait until Second Encounter HD comes out and get a special package instead. Because I for one wouldn't pay more than $5 per game, I've already played the original and Second Encounter years ago, I don't feel like spending more money on games I already bought. I haven't bought SS HD yet although I admit I was tempted at first, but all I had to do was to go on YouTube, watch a couple of HD videos of it for around half an hour only to entirely lose all interests in it.