You can buy external graphics cards that run from USB, but they would be very slow/laggy (ie, only good for text display and "office work" - no videos/games) and may not even support a resolution as high as 1920*1200. So two external monitors is a no-go.
If your Macbook Pro is the latest generation with mini displayport, the maximum screen real estate practical for you would be if you bought a 30-inch screen (you will need the mini D.P. to dual-link DVI adapter).
But using a 17-inch Macbook Pro and a 30-inch display in dual display mode seems ridiculous and impractical in itself, the difference in screen size, DPI, and resolution is just too much to be useful in most situations. I prefer just using my 22" display by itself rather than with my Macbook in dual display mode.
My advice? You probably don't need more than one screen. I find that juggling windows between screens is just as much of a nuisance as switching between them on one screen, let alone the desk space multiple monitors takes up. If you want a bigger screen or more pixels than your Macbook Pro has at the moment, my advice is just get a single 24 or 30 inch display. For 24 inchers, the Dell U2410 is a good bet because it is anti-glare. All the 30-inch ones are pretty much the same in terms of image quality, so it would depend on whether you wanted good looks with the Apple display or more features/ports with a Dell or HP.
So, in short, grab the Dell U2410 or splurge on a 30-incher if you're really serious and can afford such a ridiculous screen.
Sorry for the lecture
. Hope it helps.