Yes, but only to a certain degree. The most powerful card compatible is the 64MB nVIDIA Quadro4 700 Go GL, part 6X935. Alternatively, and much more cheaply, the 64MB ATI Mobility Radeon 9000, part 4U284 is also compatible (however note that this one will not work with some Hitachi and IBM SXGA+ screens). Your only outlet of purchase is through Dell spare parts. Due to the expense of these cards often people find it more economical to just sell their old laptop and splurge on a new one for an overall performance boost.
Stu