My initial thoughts would be that your present CPU would bottleneck a 8800 Ultra, & the PSU may not be powerful enough. Also, as you have a 32-bit OS, only 3-3.5 of the 4Gb RAM will be recognised/addressed by the system.
Any how, the best place to ask these questions may be the Dell forums, in particular the Upgrade section click here.
Don't forget to ask if you'll require a new PSU as well, & what kind of power connectors/adaptors are necessary for a 8800/ATi equivalent. G