I have yet to fully play the first one but I love me some hack n slash goodness. From what I saw they did a good job on the first game, not exactly sure how the MMO will fair but I think that depends more on if they go pay to play or free with the options to purchase DLC.
To echo everybody else, without a doubt start with SotN. Rondo of Blood laid down the foundation but Symphony was the blueprint for almost every Castlevania that followed after.
The brief dips into 3D on the N64 and PS2 are a toss up, some people enjoyed them and many hate them. I say if you find one that appeals to you give a try, but only if you can score a cheap used copy. If you already have a DS or two to play all the handheld games them I'd say just stick with those for now.
This is all well and good but with the new Marvel vs Capcom game on the way too I get the feeling Capcom is going a little overboard. With the time and money they are putting into these VS spin offs they could at least bring Darkstalkers up to date or some other highly anticipated Capcom series back from the dead.
Then again they did try with Bionic Commando and look how well that turned out. I guess this is Capcom's way of playing it safe, so I suppose it's not THAT bad.