I bought orange box from ebay for $53 (2nd hand copy), so pretty much the same price as you. I haven't played half life 2, but heard so many good things about it, and seen it consistently get high 90's in reviews. I even think it's the highest rated pc game of all time (metacritics), so I really had to play it. Plus on top of that you get episode 1 and episode 2 which are basically expansion packs to the origrinal, then you get portal which I've heard is incredibly fun and addictive, and on top of that you get multiplayer in the form of team fortress 2. I've heard it's an instant buy for those who haven't played hl2 offering the best value for money in any game ever.
For $50 not sure why you're so indecisive, it's not like you're about to get married or have a baby lol...
You mentioned graphics...sigh...
I only just started playing it today, and I wanted to play episode 2 first coz I heard they updated the graphics for it but it wasn't stellar but within 10 minutes of playing it I forgot completely about the graphics as the gameplay/story sucked me in. Incredibly fun is all I can say. must play imo. I could go on and on about how fun it was (for the 20mins I played it) but all you need to know is that, yes, it's a definitely worth purchasing. Stop reading and go order it from ebay, now.