Heyyo,
HuntKillaz, nice choice with HP, for a big company selling comps at least they're nice aboot 3rd party upgrades.
but I checked the specs, that 128MB of ram won't cut it, I believe even with a new videocard like that. The cpu's a little weak too, just a little celeron.. might wanna upgrade that too if possible...
It also doesn't mention your power supply, make sure it's at least 300 - 350 Watts or it won't be able to hand that videocard and you'll need a new powersupply to go with it...
If I were you, for HL2 go ATI cause it's ATI Optimized. But if you're gonna get a big card like that? I'm pretty sure it won't matter too much.
[EDIT]
Dudes, we should start a stress-tests thread, where we post our comp stats and our CS:S video quailty settings and our average framerate results so we can see how well different setups run on the source engine.. who's up for this idea? I am.