We can't tell you without knowing what your previous video card was. However, I can tell you this.
Your CPU is really, really awful by today's standards. It is totally insufficient to run modern games. It is absolutely going to be the limiting factor in your gaming experience.
Also, the 9800 GT was great in its day, but is at best sub par today. That's not necessarily a bad thing, because your CPU is going to bottleneck the hell out of more powerful video cards.
All in all, I'd say your computer is extremely weak for a gaming computer in 2012. You'll need to run at medium to low settings, probably at a resolution no greater than 1280x1024. 1680x1050 might work in some games, but 1080p is out of the question.
If you want to upgrade your computer some more to get better gaming performance, you can post more detailed system specs, the games you want to play and the settings and resolution you want to play at.
Specs we'll need are exact motherboard model (so we can find out what CPUs you can put in it) and more specific information about your memory like its speed.
Edit: Just noticed that the old video card was in the title of the thread! Ignore my previous comment about not knowing how much of an upgrade it would be.