Is this upgrade a good idea

I think the 1950 will work great for you with Bioshock and Crysis. While you won't be able to play at the highest settings, it will be playable at medium settings most likely. Your processor is ok. I would imagine you don't want to upgrade the platform to a newer amd or intel with PCI express so the 1950 pro and some ram is a good idea.

I think just buying 1 GB of RAM will do you. You could pobably keep this upgrade around $150-200 with just the card and 1gb of ram. Grab that PSU also.

Check the dimensions of the card on newegg and measure the dell case to make sure there is room for the card.

edit--------------------------------------
You may not need a new PSU. Try it without the new one. I can't remember if the 1950pro requires power, it might just use the floppy power connector, I cant remember. See if the system works without the psu. You might save some money. Either that or buy a nice PSU and use it for now and when you build a new computer in a year. Get a 600W one if that's the case!

Jam it back in, in the dark.