Haha, I used to be using a 2.0GHz Pentium 4 with 1024 MB of RAM and a GeForce 6200 on PCI. When you've got a PC that crappy, you eventually learn some great things to improve framerates.
Well, actually, now that I really look at your computer, it sounds more than fit to run the game great. It really should be able to tear through this game, considering it's a lot less expensive on your system than, say TF2 or Left 4 Dead, since it doesn't have 5 shader layers all the time.
I would recommend turning down your resolution, I can get a 20-30 FPS difference by playing at 1024x768 instead of the recommended 1280x1024 on my 9500 GS.
Another thing to try would be going into your game launch options under *right click on game*>properties>set launch options... then, add this:
-threads 2
this should ensure that you are utilizing both of your processors.
But of course, it could always be some crazy Vista problem.
If you want to try some other things, check this out, it helped me a lot:
http://www.tweakguides.com/HL2_1.html
