Buy a power supply that exceeds the minimum recommendations by a fair amount (20% or more). You can add the 12V voltages across rails if it's a multi-rail PSU. Make sure it's 80+ certified and tested for 100,000+ hours. This will at least get you into the range of better quality PSU's.
When you've chosen a few that you're considering do a reality check to see if the numbers are being fudged or are reasonable. V x A x efficiency = W. None will be perfect but those that come closer are likely better.
Also look at the power connectors to make sure there are the right ones for your current and future needs (floppy, SATA, molex and especially 6 pin, 8 pin or 6 + 2 pin GPU connectors and 20, 24 0r 20 + 4 pin main power connectors).
Better brand names include PC Power & Cooling, OCZ and Seasonic among others.
Remember, especially with PSU's, you get what you pay for.
If you want to go to a lesser video card check the 9500 GT's at newegg. Get a GDDR3, 512 Mb with the fastest memory clock you can find. It'll run the game fine but may still require a PSU upgrade.
If you try to run one of these cards with what you have you'll likely burn up either the card or the PSU.
One final note: Dell, at least in the past, has liked using proprietary power connectors for their PSUs. Make absolutely sure that you have a generic PSU connection at the motherboard before buying a new PSU. If you don't I know that PC Power and Cooling makes a 500W PSU specifically for Dell machines.
