The fact that your hardware is outdated for the newest games (graphics and audio support) is no excuse for sticking with a crappier engine. HL2 running fast on the highest settings is not a sign of good programming. It's a misuse of resources. This is why HL2 looks (and sounds) like garbage compared to BF2 on the highest settings in both.
For the record, I can run BF2 on all highest settings except AA, which is forced to 4x, with no hiccups, perfect looks and sounds. And judging by how long it takes to pump out each version of this mod (and that's even on the same engine, let alone changing engines), by the time Inf comes out, my bet is that most people here will also be able to run BF2 no problem on high, if not highest settings. You don't make a game/mod great by basing it around a slow/outdated userbase. The players will fill in around the great game.
There are more than 1 million registered users playing Multiplayer BF2 - we'll say ~750k of those active. And that's after a very short time. If BF2 is as slow and crappy as you say, I doubt that # would be so high.
I'm not trying to make a hardline choice here even if that's what it sounds like. This is not Windows v. Macintosh.
But you have better (i.e. more up to date) graphics support in BF2. Greater audio support (Creative X-Fi "Ultra High" setting). And many of the features the team is experimenting with now are already implemented and ready to go with slight modifications, are they not?