HL2 has a great backstory, and if the second is as good as the first in that regard it'll be a ton of fun - amazing new eye candy aside.
If I were only allowed to play one or the other, I'd choose HL2 in a heartbeat. The reason I love DOOM is because I have such fond memories of it when it first came out (remember Castle Wolfenstein and Spear of Destiny?). Particularly DOOM 2. I used to work for EA many moons ago and we used to have these ass-kicking deathmatches on the company LAN. You have to remember that up untill that point, it'd never been done. So it was a hell of an exciting experience. We'd literally play all night long. People screaming and trash talking each across the office. It was a blast!
Still, in spite of that, HL the first shooter to become something more than just a shooter. It was one of those amazing firsts, sort of like Fallout. It transformed the genre and moved it forward. I hope HL2 will do the same once again.
As for having a POS PC, get a better one. I know you're young but you can always get a job in hospitality as a busboy. When I was 16 I got a job as a busboy in a najor hotel restaurant, was great cash for the time compared to my friends who were working fast food jobs and the like. You'll make 3 times as much doing that and most of it cash.
That could put you in a position to get yourself a nice new rig come the fall when all the PCIE stuff starts hitting the market. Can hardly wait for PCIE stuff. It's killing me waiting for it.
Either way I'm down with Dewm 3 and HL2 (color me in for Vampire the Masquerade too! Can hardly wait to see what Troika does with that.)
"So you gonna' talk to me or we just gonna' keep bullshiting each other? Breakin' my balls, Chuck."
"And then, suddenly and without warning, it turned into a real-life case of hungry, hungry hippos."
- Stephen Colbert