If the Giants beat Cliff Lee (it will take a gem from Timmy, so not entirely unexpected), the Giants will roll through the Rangers in 4 or 5. The Texas "faithful" are putting most of their eggs in the "Cliff is unbeatable" basket, so it's more of a must win tonight for them, especially psychologically.
If Cliff dominates, and the Rangers win handily (> 3 runs), the Giants will likely get steamrolled in 5 or 6.
If it's a nailbiter and Texas wins (something like 2-1, 3-2), the series is up for grabs and will go 7.