To set matters clearly, the 9800GT runs at a stock speed of 600MHz core, 1500MHz shaders, and 1800MHz DDR3 memory. The GTS 240 on the other hand, will run at a 675MHz core, 1688MHz shaders, and 2200MHz DDR3 memory. Both cards still have 112 shaders and a 256-bit interface, however it might be possible that Nvidia is implementing the same DDR3 chips found in the 9800GTX+ on the GTS 240 (opposed to Samsung K4J52324QE-BJ1A 1.0ns GDDR3-2000 ICs on the 9800GT) due to the noticeably higher frequency.
On another note, it is also safe to assume that the GTS 240 will not have support for HybridPower due to current market trends.