First, if you purchased a Radeon card in the past year or so, you may already own Half-Life 2 and not realize it. Many Radeons shipped with coupons for the game, and those coupons can be redeemed instantly via the Steam distribution system. I got my copy that way, and the HL2 files are downloaded and encrypted on my computer, waiting to be unlocked. If you've forgotten whether your card came with a coupon, now would be a good time to go check.
Heck, I'd recommend purchasing via Steam for anybody with broadband, but then I also kind of expect the system to be suffering mightily in the next few days.
Also, we have already tested performance in Half-Life 2's Source game engine extensively around here. For graphics, go see our Source engine benchmark fest or, for even newer cards and drivers, see the video stress test on this page of my Radeon X700 review and the Counter-Strike: Source benches here. For CPUs, we have Counter-Strike: Source benchmarks right here in our P4 Extreme Edition 3.46GHz review.