Need Video Card Help

OK, folks. I need help of the kind that I think some of y'all can offer.

Three years ago, I bought my computer from CompUSA. At the time, the computer was pretty well loaded... with a whopping 64 MB Video Card (NVIDIA GeForce4 MX420 graphics card) and 512 MB DDR SDRAM.
That was three years ago... nowadays... nowadays, my computer tends to meet minimum specifications for the games I want to play and some of my games tend to get slow (both slow to load and slow to play, depending).

So.. what I want to do is begin refurbishing my computer. My computer doesn't run too slow and I don't have problems, for example, hopping in and out of Word or Excel, so I suspect it's not actually a RAM problem.. I think it's that with the new 3D games I'm playing (Sims 2, Zoo Tycoon 2, etc.), the 64 MB Video Card just isn't up to snuff anymore.

So.. I want to replace it.

The problem is.. I don't know a damned thing about Video Cards. I don't know HOW to compare and contrast and decide which is going to get me the most bang for my buck.
So... could someone with more knowledge of the intricacies of computer parts please instruct me as to the difference between video cards and how to tell which one is right for me?
Please?

Thanks!