GeForce 9800M GTS SLI is a solution of two GeForce 9800M GTS put together using NVIDIA'S SLI technology.
Check the page of GeForce 9800M GTS to know more about its chip.
As usual, SLI relies a lot on proper driver support and may suffer from micro-stuttering in lower frame rates (below 30). Benchmarks indicate the performance is overall, up to X% better than a single GeForce 9800M GTS performing by itself but at times (depending whether or not the 3D game supports SLI or in the graphics driver) it performed worse than a single GeForce 9800M GTS.
A few demanding games will run at the highest settings with fluent frame rates (30+) but overall most games will require reduced settings to be played optimally.