Very good choice, IMHO, the half life series has the best story I've ever seen in a computer game.
I strongly recommend playing the original Half-Life before you play Half-Life 2 as to get the full effect of the story. Don't worry, even though the graphics aren't amazing NOW the gameplay is still incredibly fun.
Then you have a choice; you could play the half-life expansions (which I didn't, but I feel I should have) or you could start the HL2 series. I recommend playing HL1
pposing force and blue shift because when I didn't I felt like I may have missed some important info regarding the story.
But, they aren't 100% necessary. If your getting tired of the old GFX, then move on to Half-Life 2. Not Half-Life 2: episode one, that comes AFTER Half-Life 2.
I don't know what Valve was thinking when they named it HL2: episode one instead of HL3: episode one.
Anyways, they are really, REALLY great games and run on the amazing source engine which is also IMO the best (most efficient for its looks) game engine out there as of now.
Sorry for the long post, but I really love these games, and watch out Oct. 9th for HL2: episode 2!