Damn.... it would suck to be in your position if you just bought that 5900 at full price hoping it will play HL2 and Doom3.
Well, welcome to the real world. Here, we read our own posts and think how other would react to them. Here, we look at facts and not what they should do.
By your ability level, an Athlon XP 1700+ clocked lower than a Pentium 4 1.7Ghz should perform worse, right? Wrong.
Game developers always find a way to make games work on hardware that isn't designed to run it, right? Wrong.
It can go either way for the second question kind of. I'm sure they do try and get things like FSAA running nicely, but there is no guarantee. If they don't get it working, trust me, they will still have a great game to show. Not every gamer uses FSAA at all or even AF for that matter. I wouldn't go bashing how they say it will work on the Radeons with some work arounds but not at all on the Geforce FX. It may have to do with hardware. I bet that had it been the other way around, with ATI's cards not being able to run FSAA any way possibly, then you wouldn't have given a crap about it.
Just remember, the game is still expected to make a nice display and if FSAA is the single feature they depend on to make it look good, then it needs more work. Many games are still great without FSAA because the average user won't be able to run it on a Radeon 9200 and lower.