Well, assuming you want to play current games on that thing you will need more upgrades than just a video card. Yes, your ram is a quite low, you will want to add at least another gig. The bigger problem is your processor which is very poor by today's standards. Fortunately your motherboard is AM2 and can use much better processors. Here is a list of what it can handle; http://www.cpu-upgrade.com/mb-ASUS/M2N-MX.html
I would recommend a dual core of at least 5000+
Basically you probably shouldn't bother with a new video card unless you can afford a new processor and another gig of ram as well.