The PS and Northbridge is probably $100 (the ps is a necessity), if you have the 6 cell battery the 9400 series came standard with, you really should have the 9 cell. DELL hits you for like $249 for that which is absolute crazy talk, but is necessary. The GTX needs the juice.
A 7900 GTX with 512 MB runs about $415 or so, the 256 MB is cheaper (I think DELL has it for like $365), but if you're going to go for it, you might as well go all the way. I wouldn't recommend the 7950 because of that weenie roast heatsink that DELL uses (you can see it on the following links).
Look at these articles for the mod:
Legit Reviews DELL Upgrade
Upgrade your 9300/9400 DELL