Ok so i first played HL2 on a 2.9Ghz processor Pc with a crappy soundcard and an ATI graphics card. However the memorey was only 256 and so to get the best performance everything is set to low and it jumps a little but its withstandable. Now when i get my net connection up i will be playing on a 2.27Ghz porcessor with a 64mb DirectX9.0b NVidia Ti4200 Card. Memory is 512 and i got a nifty soundcards. Surely that means it should run smoothly on medium and maybe even high deatil levels right? Right?
it should definately run fine on medium detail at the least. what kind of bus does your new vid card use? if it's agp 8x, then i guess you're good to go, m/b willing. the processor certainly is more than enough in the number crunching area. i've got a system that's mostly 4 years old. the only things i've changed in the past 2 years are the video card (ELSA Gladiac Ultra 64 MB to MSI GeForceFX 5800 128 MB) and my processors (dual 1.2GHz Athlon MPs to dual 2800+ Athlon MPs), but my video bus is agp pro (4x) and the card is a non-ultra, so it wasn't as much an improvement over the old GeForce2 Ultra as i had thought it would be - aside from the additional hardware niftiness support for reflections and such. i run hl2 at high detail (doom3 at ultra), with full water reflections, 1600x1200, trilinear filtering, and only experience a few spots of slowdown in big fights. the big kicker is the antialiasing (and anisotropic filtering on some older cards) which i can't run above 2x without watching a slideshow. i've got 1024 MB of old PC2100 ECC DDR SDRAM and three IBM Ultrastar 36GB Ultra160 hdd's. this thing was a beast when i built it, but now you can pay 800 bucks and beat it for some things. i used to brag about the 10000 rpm harddrives, but then 15000 rpm ones came out, and Ultra320 on top of that. beef up on the ram as much as you can. be wary of antialiasing. even the basic water reflections are nice enough, in my opinion.
Since you didn't name your old graphics card, it's hard. Something will run at medium and some on low. Your graphics card isn't the best out there but you have enough ram and CPU power.