If you don't have a decent enough Power Supply, you will also need to change that to power your new card.
I'm guessing your machine is still using Dell's weak OEM PSU.
Perhaps, if you could tell us how much you're willing to spend on the upgrade, we could recommend a PSU + GFX Card combo for your PC.
I have a Dell Dimension 4600 that is about seven years old. It was running fine. I bought a Samsung B2330 monitor connected via DVI but I have issues with my old graphics card and this monitor in terms of resolution and speed. I am running XP sp3. Intel Pentium 4 3.4GHz.. The current card is a NVIDIA GeForce FX 5200. Should I simply replace the video card or is it time for a new desktop? If a new card alone is OK, I would appreciate suggestions. I do not do any high end graphics or gaming. The most intensive graphics usage is looking at medical imaging on line. Thanks in advance for your guidance.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic