External GPU for MacBook Pro?
The answer is no (correctly answered by @JFW), this is not possible using Firewire because the protocol simply does not provide the required bandwidth. However, when the day comes you purchase a machine equipped with Thunderbolt, continue reading.

Sonnet recently announced the release of several "add-on" components that make use of a Mac's thunderbolt port. These include a RAID assembly, an ExpressCard, and even a PCIe expansion bay, that can accept any PCI Express 2.0 adapter card.

Sonnet's product listing: http://www.sonnettech.com/product/thunderbolt/

Village Instruments one-upped them by committing themselves to manufacturing exactly what you're in the market for. However, before you jump for joy, you might want to head over and have a read. While the card will outperform integrated cards provided by the lower end MBPs, they still aren't up to snuff compared to some of the bigger cards available for PCs.

Head on over and have a gander: http://www.extremetech.com/computing/92160-external-thunderbolt-graphics-card-for-macs-will-not-blow-you-away

So even with Thunderbolt, it is possible, it may not give you the bang for your buck you're hoping for.