Yak, speaking from personal opinion and experience, you'll most likely just have to upgrade your video card and be able to play Doom 3 and HL2 on fairly high settings.
I know what the specs say, but frankly they're wrong. Since they don't test the game on every possibly configuration.
My system to many games is classified as a "low-medium" end system, yet I can run every game that is currently out on their highest settings with no problems.
My specs are not much higher then yours, except for the video card, which is now almost 2 years old.
Specs from game companies is not a real indication of what system you'll need, they're always overshot.
And ... on a (as Orph would say) taboo copy of D3 it runs beautifully. :smile:
I've got: Athlon which runs at about 1.9-2.0 ghz (don't really focus on CPU speed when considering games, since it mostly effects FPS. Unless the games you play drop to noticeably laggy levels, like 12 fps, then a new cpu might be in order ... but then again, some games just lag
cough Moh
cough)
Also, your cache sizes matter too, as for your bus speeds.
512 Mb ram (400)
GeForce 4 Ti4200 (MSI if you were wondering)
My board just has a VIA KT400A chipset (shut up, Jeff

) and as many times as I've heard arguments against it being helpful, an 8X agp port.
But, honestly, the thing that will boost performance the most is your video card (obviously), after that, memory sizes/speeds, so on and so forth.
Sadly, right now graphics cards are in an 'in between' phase. Which means you'll get screwed up the ass if you buy a card right now. Wait until like September (at the earliest) to go card shopping. Other then that, if you find a cheap deal for a new p4 (we're talking 40 - 50 bucks), go for it, it wont hurt, but isn't needed.
Mixing all this together, you'll probably suffer long load times, and fairly smooth gameplay on anything (it usually gets smoother the longer and more often you play)
Hope that all helps.
[EDIT]
HL2 will run at like the lowest setting on your computer, simply because of your video card. (I'm sure it can go even lower)
How well, I have no idea. But you have a while until you have to make a decision :smile:
(September 1<sup>st</sup> my ass, EBgames!)
[/EDIT]