With a modern graphics card, Medieval2 tends to be CPU-limited (particularly as it doesn't take advantage of more than one core). Some graphics settings such as shadows and grass can hit your frame-rate through CPU calculations.
Empire:Total War hasn't yet been released. If the available demo is anything to go by, it still has a single main thread, but other lesser threads allow additional cores to do some support work. The demo doesn't seem limited by my C2D @3.2GHz (full game is sure to ask for more though!), and there are plenty of graphics options and eye candy to put your 4870 to good use (auto-detect puts the video detail to Ultra with my 9800GX2, and the small demo battles run fairly smoothly, as with Medieval2).