i have a t60p with a t7200 and 1 gig of ram in 2 512 sticks. the fglrx drivers are not nearly as good in terms of performance in comparison to the windows drivers. that being said they're still alright, it really depends on the games you play.
i'm not much of a pc gamer, i was at one point additiced to counter strike but that was several years ago, now i can't stand fps' (except metroid prime and tron 2

) but for the games i like to play it's perfect. everything runs flawlessly (games like rrootage, tumiki fighters, neverball, gltron, cave story via wine and emulators like fceu, zsnes, mednafen, sdlmame). i also run xgl/compiz with gnome and it's very stable, haven't had a single problem with it.
i picked up the demo of doom 3 for both linux and windows tried both. it's hard to give what my fps was in each, as it tended to jump around in both, but i believe it averaged at around 60 for windows, and 30-40 for linux. under linux the frame rate jumped around a lot more, some rooms it would be great, others really jumpy. but for the most part the game was playable if not sometimes choppy. i was playing at 1024x768 at medium quality with all the items in the advanced options turned on. also i found lowering or raising the quality just barely affected the games performance at all and i couldn't see a difference in quality between any of the settings. also lowering the res to 640x480 helped the most, giving me at least 10 extra fps most of the time, it made the game feel much smoother as well. but still, playing the game at this res and at low quality was like playing the game in windows at 1024x768 with ultra quality in terms of smoothness/consistent framerate.
on the plus side the loading times were much faster on linux..
edit:
i forgot to mention i have the t60p with the firegl v5250. also i played the game again in linux today and it seems to be a lot more choppy than i remembered, especially when fighting the demons, drops into the 20's, and if there are >= 3 enemies 15 fps (and this is at 640x480 res at med quality)
finally if you haven't seen it already
here's a review of the latest fglrx drivers, they used the t60 with an x1400 as their test machine.