What will a more modern driver improve?

Right now I am playing a Ping TISI (8.5*) driver from about 10 years ago with a new TFC 149 (i believe) stiff shaft. The total length is ~46". Any way as a little background about me I started playing again this summer after a long hiatus(from when the driver was newish). I hit the driver decently and hit near the center ~70% of the time. My usual shot is ~270-280 and straight with no draw or fade. My usual miss is a tail-off fade to a slice.

My question is what would switching to a new driver, say a G20, get me? Would a new driver get me more forgivness or distance?

It will give you both. I would say that the forgiveness is the best trade off personally.

You would definitely improve on your mishits, as already stated.  All things being equal, those dead solid perfect shots should be about the same distance.  The fitting process now is greatly improved.  You may even find that you need less loft or a stiffer/higher flexing shaft with a new driver, since so many of them have a lower and deeper center of gravity.  You can also add the adjustability factor (which can be a double edged sword) to fine tune your ball flight.  Lighter shafts and clubheads may even increase your clubhead and ball speed, though I'm not convinced that lighter is always better.  The choice of shaft flexes, flex profiles, weights, tip sizes, etc....are much greater than 10 years ago as well.  I'm sure you could find a more constant flight with a good, modern shaft without breaking the $100 shaft barrier.  Many of the modern shafts will come as OEM in the newer heads with no or minimal additional charge.

less money in your pocket