MacBook Pro 17"

My suggestion would be to compare the chipset and graphics chip on the 17" to what's available on the newer models, since that's what you can't upgrade.  You can max out the ram and either replace the HDD or the DVD drive with an SSD, either now or when the prices drop some more, but you're stuck with the processors.

I think I read something on MacRumors forum about people removing one of the existing drives and putting them in an enclosure that converted them to an external drive.