Hmm... forgot about resolution. I Got this from a benchmark site running Half Life 2:
GeForce 6600 GT - Normal = 44.3 High = 43.5 at 1024x768
GeForce 6600 GT - Normal = 43.2 - High = 41.9 at 1280x743
So it is a stable card, and this is the card ran on a AMD Athlon XP 2600+ based machine may I add so you'd expect more!
In Doom 3 at High Quality and at 1024x768, expect it to run at about 50-60fps!!! This is ran from another benchmark site running on a...well it doesn't' say :S Can't be that far off yours anyway
Expect more frames per second though on Half life 2 running off an Athlon 64 at 3000+.
Hope this helps