We have the exact same computer. I can't wait to I can build my new one, these Dells are a pain in the butt when it comes to upgrading. 128mb of RAM is horrible. Before you worry about your video card, you should have at least 512mb of RAM. Even that is low nowadays, but it will work. I have 768mb of RAM and the same video card and because of the RAM, the PC will play WoW with little lag, Half-Life 2 on low settings, UT2004 small deathmatches on all max settings at 1024x768, and lots of others.
So, with that in mind, upgrade your RAM significantly before you mess with video cards. The RAM will be cheaper and help more. When you decide to get a video card, I'd suggest a 6600GT or similar because I don't know if the stock PSU and motherboard with the 4500 can handle a 6800GS.
If you'd like to get into some of the newer games, just build a new PC altogether is my suggestion. Anyway, good luck!