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Well, it depends to what type of aftermarket throttle body you change.
If you leave the stock intake manifold alone, you are constrained to make some pretty minor changes -- a few mm diameter increase, ported a bit, etc. -- nothing else will fit the stock intake manifold. The power change will be barely detectable on a dyno, and probably not something you can really feel on the street.

If you go to a really bigger throttle body, you will need an aftermarket high-capacity intake manifold with an intake "hole" large enough for the bigger throttle body (e.g., you could go to a F.A.S.T. manifold and a 90 mm throttle body). This will make a difference you can feel::: but if you have a stock engine, you won't like what you feel: you will gain a bit on top, maybe 20 HP, but lose a lot of torque and driveability at low RPM, where you have to car most of the time driving every day. - it will feel sluggish and unresponsive in traffic, etc. The really big throttle bodies are for stroker engines or stock blocks with heads and cam kit mods, etc., -- engines that dyno around 425+ at the rear wheels.

A better approach to the intake, if your heads and block are stock, would be to go to a really good intake mod (Halltech, some other really working cold air) which would add only a few Hp stock, then put on an aftermarket intake manifold and keep you stock trhottle body or get one that is only a mm or two, and ported/polished. I would recommend the F.A.S.T. 78 mm intake - the Weiand and Lingenfelter ones are basically just metal versions of the LS6 which is not enough of an upgrade.

But if you have not put on headers, that is the single best mod you can do to make a difference you can feel - a good set of long tube headers and a cat back will make a difference you can feel, particularly in the low and middle end, but up top, too.