I like the 7600GS for machines of that level. It performs well and you might run into situations where it has better compatibility or performance with some later games that are still playable on a P4. But if you just want the P4 to run games from the early 2000s and don't care about stretching it's abilities, then some older model cards may do just as well in older games.
Period correctness can be looked at different ways. When the AGP versions of cards like the 7600GS were released they were intended as upgrades for people with fast P4s (along with Athlon64s), so really that is the period they were always targeted for, even though they are newer than the systems in question.
Another possible task for a P4 machine is being able to play HD video. A fast P4 is borderline for being able to decode H.264 1080p in software. I would say a 2.66GHz Northwood can probably handle 720p consistently, but not 1080p. I've noticed that Prescotts are faster at this, but still they have to work hard at it.
If you care about HD video on a P4, then it's worth considering some of the late ATI AGP cards which have H.264 acceleration. The acceleration on the HD2400 and later cards is very effective, but there are limitations in how it can be utilized. My experience with an AGP HD2600XT has been that it will do hardware decode if I play a saved file through the MPC-BE player, but it's not accelerated in VLC or in a web browser. I think the HD4xxx cards are less limited, but they're expensive.
In terms of gaming, I've only seen the advantage of the HD2600XT vs 7600GS show up in shader heavy games that you wouldn't normally play on a P4.
