That depends on the resolution you choose to play with.
The 17" is capable of gaming with 1280 x 800, the same with the 15".
But 1920 x 1200 will be slower than 1440 x 900 or 1680 x 1050.
If you're serious about gaming, get a desktop PC. Otherwise, your best bet is a 15" lo-res i7.
If you're comparing the 15" 1440x900 res to 17" 1920x1200, thats a significant increase in resolution. If the options in graphics stayed the same, across the board I'd say you're looking at around 15-20% hit in frame rates... possibly even more depending on the game.
This is based on what I've seen on mainstream graphic cards performance testing when increasing resolution without changing any graphic options.
Oh I know, but in my experience, playing at any resolution other than native makes things really blurry.