The Nvidia FX series was a pretty big bust for Nvidia back in the day. Plus your 5700 is the LE version, which is fairly low end. Your processor isn't that bad and I think you will see a big bang right now if you got a video card. However, it's almost a toss up with the processor.
Adding more ram? Well, you're running DDR, not DDR2. DDR is older and is currently more exnoodleve than DDR2, and will probably be from now on. 2gb of DDR2 runs about what only 1gb of DDR costs.
I guess the best way to put it is your computer is getting dated. You still running a motherboard an AGP slot and cards for this are going to become harder and harder to find.
I would sit down and maybe work out a budget. Would spending $100 be good now to get a little better performance? Or should hold onto that money, wait a few months and get a whole new system? Myself, I generally tend to lean towards the latter; however, I have buddies which would rather do the former.
From my perspective, AGP has been on the way out for a while now and their cards are becoming more expensive than their PCI-E counterpart because they are not manufactured in great numbers anymore.
I'd think about what your budget is now, or what it could be in the future.
