Actually in my experience, 256mb and above are ok to leave T&L on.
I have found that the biggest hit on performance hands down is the LOD of the models in use, the sizes of their textures, and the amount of ships/objects on screen.
If any of you are like me and have a ton of different ship mods installed from the different what I call "eras" of BC modding, then depending on the ships/objects you play with will be a huge determining factor on your overall quality and playability.
For instance, whenever I play with a bunch of older p81 ships (I keep then around for nostalgia... and also for the fact that they're a great improvement over the original BC ships) I can play everything at maxed out settings and even those old ships from a distance don't look too bad.
On the other hand, when I play with a bunch of DJ, LC, CG, or any other high res models on screen at higher resolutions then I definitely start to take a hit on quality/performance... and I have a pretty powerful system.
It's all about balance.