SirPrimalform: Ooh, make sure you play Aria of Sorrow after HoD and The Lost Age after Golden Sun.
ovoon: Yea I intend to. Do you think these sequels are better than the originals? This is my first Castlevania game by the way, and I absolutely love it.
I do think the Lost Age is the better of the two (GS1 just feels like a prologue once you get to 2).
I think Aria of Sorrow is better in some ways. It's not a sequel per se (few Castlevania games are direct sequels) but it carries over a lot of mechanics from HoD as well as some from Symphony of the Night (the PS1 game that provided the template for pretty much all 2D Castlevanias that followed by basically being a Castlevania themed Metroid game).
If you end up liking Aria of Sorrow, there's a direct sequel on the DS called Dawn of Sorrow which is also pretty good. Portrait of Ruin follows that and is a pretty solid game, but not the best. I've heard excellent things about the last DS one, "Order of Ecclesia" but I've not played it myself.
Also, Golden Sun has a third game on the DS which I have yet to play.