I really don't think you have anything to worry about. If ID released a game that only played well on extremely high-end systems, it wouldn't make any money on the deal nor would it attract as many 3rd party license buyers (where the real money is at).
The ti4600 will be perfectly fine for D3 and HL2. Valve has publicly stated the game will scale as low as a TNT2. Anything from a GeForce3 on up card will be able play the game with plenty, albeit not all, the eye candy turned up.
Regarding Doom3, if the very unoptimized alpha demo can play reasonably well on my GeForce3, the final, polished build should play even better on a ti4600.
FYI, can you guess what video card is running Doom3 here?
or this one?